The physical USB connector can fail, clean it up and straighten anything which is bent.

Windows sometimes fails to properly eject/unmount the device. Not so much in post-WinXP, but it still happens. When it does, the device's file system is corrupted by mismatched FATs; one copy wasn't properly updated by the "helpful" drive caching (which can't be turned off, regardless what Windows claims). ScanDisk can easily repair this problem in unrecognized flash media, although the most recent "unsaved" changes are usually lost.

U3 problems can be corrected with some kind of "USB firmware tool" software. You may need to disassemble your device to learn exactly which controller chip is used (from part markings).

Flash memory is a type of EEPROM, it ultimately fails after some (large) number of write cycles. Better flash products use better part specs and clever wear-leveling algorithms to extend product life, they can also isolate bad blocks (and activate backup blocks!) to prevent further data loss or loss of capacity. Cheap flash products just slap together cheap parts any way which works. Some of the worst designs accelerate failure by simply rewriting to the same addresses every time the FAT (file contents) are changed, using one of these as a cache drive can burn out the flash block within hours.