Items from chests not stacking

I went into my chest in my MCPE survival world and was trying to stack items. I had about 30 cooked cod and another 15 cooked cod, but they would not stack. They would switch places in the chest if I tapped on them, and would not stack if I double tapped on one. I made sure they were both cooked cod (which they were) and the problem is still happening. It is also happening with my cooked salmon.