I'm CERTAINLY no expert, but I would try adding something that is a known favorite, like pumpkin or watermelon or horse manure. Maybe they just don't get excited about apples. Mine don't seem to LOVE apples either, but I do, so they keep getting my leftovers. :)

You're not giving them enough. Just mix up a mixture of food for them, and feed it. A lb of worms should eat a nice amount of food every few days.

They're surviving by eating their bedding, so they're doing all right, I would guess.

Try feeding some other kinds of food and see how they do.

Usually I mix stuff, the apple was just an example.
I tried to add a little bit everyday, but the tray become full and for sure I go on overfeeding them.

Are you optimizing the food first? eg cutting into small pieces. Freezing first.

Yep, that's why I have no clue.

An apple peel every two weeks isn't nearly enough food. And peels of almost any kind break down slowly. I have fed worms a variety of peels, and most do OK, given enough time. I will second the advice to find something that is more of a worm favorite and feed it in larger amounts.