We've been in our house for 4 years. We like the tiles in the kitchen but it looks like they've been a really bad DIY job, not completely flat, not a huge deal BUT there is movement in quite a few and some definitely sound hollow, some have cracked as I've stood on them or when something has been dropped on them. I have a stack of spare tiles which were in a cupboard, is it worth trying to replace or is it likely that they cement is failing and this would just be a sticking plaster?