Yup, have had this problem. There are several issues: material not what i expect, poor fitting pouch, back strap too narrow/wide, thickness of material not to my liking etc...
Now most of the issues are usually down to the buyer - i admit that my not liking the 2xist y-backs due to the strange pouch shape and too-thick material, or the Cocksox thong with a really big back and strap and pouch too torpedo-like, or cover male material though snug like a Joe Snyder had no stretch etc...
Most of the time i find a place in my rotation. Some thongs are meant for short term wear (dates!), others for daily...