Also depends on where you're getting the cards from - amazon UK was selling a batch of fake sandisk cards; they obviously didn't know, but these things present issues over time. I contacted Sandisk directly and they simply replaced the cards (128gb uhs1 ).
Also, are you using class 10 cards? WP has issues off loading apps to other speeds, for whatever reason.
And yeah, you can just format the cards.
I've got a couple of android phones (all flagships) and they've all presented issues with sd card at one time or another. Even had problems with BlackBerry, both bbos and bb10.
Hdds and ssds get knackered also: Just get a replacement if need be! Good luck!