If you did not wash/ shrink your fabric before cutting it - it is possible that the pieces shrunk unevenly. Sometimes fabric shrinks in only one direction - and if you steam pressed it, that could cause shrinkage.
I also prefer to (usually) work with "naked" fabric - it has been washed, ironed, and no starch or sizing added to it because if it hard to handle then, I prefer not to fight with it.
But there are many on the board that don't wash, and do starch.
Or wash and starch.
Were the original pieces in the layer cake all the same size? Sometimes there are size variations there.