I'm willing to bet you're using egg foam for leavening which is notoriously finicky. Despite protestations that it can't be overmixed, it also can't be undermixed! So make sure it's well incorporated into the batter before loading up. Batter color should be even.
For other considerations, similar rules apply if you're using chemical leavening: your tins/cups/pans need to be spotlessly clean. This is my method: Insides are then lubed up to the top with butter or shortening. The insides should then be tossed with a thin coating of something, either sugar or chocolate powder (basically some dry substance). Load your batter in to 2/3 the height of the pan (tends to be roughly the same regardless of the size of the pan for me). Do so very neatly so as not to spill batter and muck up the pan. Also give it a slight few taps to work unsuspended air bubbles up to the surface.