When you say "frozen", I'm assuming you mean "thawed" (just making sure)?
Sometimes it takes a bit of time, but keep at it. One food our fish find irresistible is salmon. Most grocers sell 6 oz pieces of salmon, tuna, swordfish, etc for like $3 or $4, and we get whatever is on sale like that. We cut them into larger chunks and freeze them on a plate, then you can toss them into a baggie in the freezer and thaw them and cut them into bite size bits as you use them.