White streaks or residual washing powder on laundry after washing
If you have white streaks on clothes at the end of a wash cycle they are most likely to be undissolved detergent. Sometimes you can get them out by sponging. If you can't, and your washing machine has a rinse only programme, use that, or put them on a full wash cycle without any detergent in.
What causes these streaks?
There are several possible causes –
Overloading: If you put too much laundry in the washing machine some of the detergent can get trapped in folds of laundry preventing it from dissolving properly (Overloading the washing machine?). If you use a detergent ball or net placed in the drum it could get trapped in folds of clothing preventing all the detergent from dispersing properly. Make sure it's placed right at the top of the laundry and to the back of the drum.
Not using enough detergent: This is counter-intuitive, but if you don't put enough in you can get a film of whitish residue that can be mistaken for undissolved detergent. This is because some detergents react with minerals in hard water so you need enough of it to soften the water adequately. Always use the right amount for the level of soiling and the hardness of your water.
Low water pressure or dirty dispenser drawer 
This is only relevant if you put detergent in the soap dispenser drawer. If you have low water pressure then not all the detergent may get flushed into the drum on the initial wash fill. This can allow some to be left in the drawer to drip onto the laundry during rinsing. If this is happening and you can’t improve the water pressure I would switch to placing it in a dosing container (provided by detergent manufacturers) on top of the laundry.
Another alternative to prevent this problem would be to switch to liquid detergent – but liquid detergent does not contain any bleaching agents and can result in mould and gunge in the washing machine if used exclusively.
The photo on the left shows a pair of dark socks with white streaks on after being washed. The streaks can be difficult to remove and the laundry may need another wash. The other picture above shows the actual soap dispenser drawer from the washing machine used to wash the socks and some of this detergent was running onto the laundry, possibly during spin.
Also, this particular washing machine has a hot and cold fill, so if detergent is put in either side on most washes it will still be washed into the drum but the user has been placing detergent in the pre-wash section instead of the main wash side. This compartment is flushed with cold water (for pre wash and rinses) so it's resulted in bits of the undissolved detergent getting flushed into the drum during the rinses. Always put the detergent in the correct side of the soap dispenser drawer according to the instruction book.
Marks caused by gunge from door seal or drum getting onto laundry
This photo shows streaks appearing over laundry after being washed. A careful examination showed the streaks to look more like gunge than undissolved washing detergent.
Lifting up the door seal fold revealed it was caked in thick, limescale and grease and this was the source of these streaks. The user had been seriously under-dosing for some years after moving from a soft water area to a harder water areas and was not putting enough detergent in the machine to prevent limescale and gunge slowly building up. See this article for more details – causes of grease, slime and black mould inside washing machines
Research. Don't waste money with bad purchases. Which? have an excellent no-strings trial offer - 
Can we claim a refund from the seller of the washing machine (Hotpoint) if it is leaveing detergent after the clothes have been washed. They (sellers of the machine) are saying because their engineer is saying it is mecahnically okay we can not claim anything – surely this cannot not be correct as the clothes are not been washed properly one of the main reason why you purchase a washing na=machine for.
KV: If poor washing results can be shown to be caused by a fault then of course you have an issue that needs resolving. Are you sure the white streaks aren’t caused by any of the things mentioned in my article? None of the causes I list are the washing machines fault. I can’t think of any cause related to a fault on the washing machine other than if the drum wasn’t turning on wash maybe.
I have a 8 year old Hotpoint WD640P washing machine and have an insurance costing £120 – repair men have been out twice and can’t find a problem with the machine and say it must be the powder (which I haven’t changed for 8 years) but my clothes are streaked with white powder – doesn’t matter if I put a few items in or a whole load, the drum frothes up as if I have put huge amounts of powder in. I have tried changing from Daz, to Persil and now Ariel – same result. I am using twice the electric, twice the time and twice the water and feel like I am paying insurance for nothing
It seems on reading the blogs that over foaming is a common problem which engineers are reluctant to admit. there surely has to be one predominant cause, my guess being ” lack of water ” It is very frustrating having the problem, which is made worse by not knowing the cause or solution, I have been waiting for manufactures (or hoping ) to realize consumers want more water in the washing machines only to learn that they are actually now putting less in. so my hopes of seeing a machine or detergent that states “will not over foam ” are futile.
I am getting increasingly itchy skin fro left in detergent. How is using less water environmentally friendly when it means we have to re-wash? Detergent left in clothes ruins them. How environmentally friendly is having to discard clothes that otherwise would have been good for much longer? Think of growing, processing, dying, and assembly costs to the environment. Another example of short term thinking. Someone should see the competitive advantage of catering for people who need an excellent wash and a proper rinse. And realise that people see the environmental and economic cost of re-washing and wasting clothes. Not everyone in the UK is short of water either.
I too have had a problem with detergent scum showing on dark clothing. It’s a new machine and I bought it because ‘Which’ said it washed well. I’ll never listen to them again. Even after TWO extra rinses, it’s still there. I have to rinse all washes by hand to properly clean all clothes. Ridiculous.