To the OP, I know this may sound weird and it may not be the case, but do you have them buckled right? I've known a lot of people to complain about their straps "slipping" or "stretching", and all it was was that they weren't on correctly. They mad the mistake of putting the strap over the roller piece, not under like it should.

I have the same straps, and they are holding up just fine. No slipping on stretching at all.

For example: