Hollow ganache

[Imjustagirlwhobakes]

I'm making my friends wedding cake and I'm using white chocolate ganache over the sponge.

I ganached the bottom tier yesterday and when I got it out of the fridge this morning, the sides of the cake are solid but the top middle of the cake moves if I lightly push on it almost like it's hollow underneath!

Do I need to take the top off and re-ganache it?? I'm worried if I put the top 2 tiers on it won't hold!

Help me please!!