What's wrong with my lemon meringue cupcakes?

The recipe says to put some cake mixture into the case, then a dollop of lemon curd then cover with more cake mixture. They come out fine and tastes divine BUT by the time the cupcakes have cooled, the cake has separated from the case .which makes it look awful. I've tried different recipes, adding more flour, putting more cake mixture (which has helped a little) but the problem is still there. Do you have any ideas as to what I'm doing wrong. I must add that all my other cupcakes do not turn out like this, only these lemon cupcakes Is it something to do with the lemon curd?