Last year I laid some peel and stick tile in my...?

kitchen. It's starting to separate. This is the first time I have ever experienced this. I cleaned the floor before laying anything down and I started in the middle. There was only one layer of tile on the floor prior to my putting mine down. Is it because I got it on sale and was cheap? Do I have to pull it up and put more down and can I put replacement tile over it? I've never used any adhesives before, do I need that? Thank you.