I've noticed lately that my 1/2 sheets are "caving" when layer them - only in the middle portion. I'm not doing anything different that I can think of & they are fully baked so I can't understand why they would be doing this. I even tried putting a flower nail in one of them just in case it would help firm up the middle of the cake, but it didn't help. When I pull them out of the oven, the middle is sunk a little, but not much & I level it off. Should be fine after that right? But then when I layer them, I notice the "slump" in the middle. It's almost like the cake can't hold it's own weight or something but this shouldn't be happening with just one 1/2 sheet on top of another! It's not real bad, but I notice it & would like to resolve the problem before anybody else notices. Any ideas of what could be causing this or how to prevent it? (My 1/4 sheets, 16" rounds, etc. are turning out just fine).
It still sounds like your cake is under done especially if it is a scratch cake. The sunken middle happens to me as well. In my case, it just was not done like I thought. When my cakes are done they dome out not in. However, I realize that on a larger sheet you run the risk of burning the outer edges or having dry cake so getting the timing down is crucial.
Guess I forgot to tell you what I do about it. I just use baking strips and let it bake a little longer.
Are you sure your oven is right on temp? It does sound like it isn't baking enough.
What size is this cake? You say you tried using a flower nail.
Could you post a pic of this ... maybe seeing it would help us figure it out.
How much batter are you using in the pan....
Just trying to guess some possible things.
Give us complete info and maybe, just maybe someone will be able to put their finger on the problem.