To expect waterproof trousers to stay waterproof?

[shufflebum]

Bought some waterproof dungarees for DS last year and they were great. Put them on today for him to use and within 10 minutes of splashing in puddles he was soaked.
I have washed them according to the instructions.
I rang the normally very helpfully company that I bought them from and because I don't have a receipt a year later they have said tough luck.
Shouldn't waterproof trousers last longer than 12 months?

If I wash something waterproof I then spray it with spray from a camping shop and it's waterproof again. I think it's Fabsil.

Not heard of that, will take a look thanks. I asked the lady on the phone about reproofing and she said she wouldn't recommend it as she didn't know what it would do to the fabric. I said I didn't really have anything to lose!

Which company were they? Most lose their waterproofing over time. Some (inc Jojo Maman Bebe) I've found you can improve the waterproofing by popping in the tumble drier on low for a bit, the heat re-seals the laminate. Others can be re-proofed with Nikwax type stuff. Some just need a good wash as when dirty they don't perform as well. Again, a good technical wash or pure soap is best, detergents can spoil the waterproofing.

My DH had a Skandia waterproof jacket. The cleaning instructions said that it could go in the washing machine but the more it was washed, the less it was waterproof. He began to spray wax onto it and it worked again. But it was too much faff and is now just used as a windcheater.