Oven problem?

[problem1234567]

I've reheated some slices of leftover takeaway pizza which were in the freezer. I warmed up the (electric) oven first for 15 mins and left the slices in for 12 mins after that.

Yet, while 75% of the pizza was hot when I went to eat it, bits of the slices were almost cold.

I suspect that something is wrong but not sure what?

Anyone more knowledgeable about ovens, please do share your wisdom.