Just so that we're using the same terminology, if you put in a hidden pleat, you should have put it in before the *first* pleat (the one next to the apron), not the last pleat (next to the underapron). Is that where your hidden pleat is?

So, I'm not quite sure what you mean by "stitching the last pleat to the apron". If you can clarify that for me, I can help! And it would also help if you could clarify what kind of trouble you're having tailor basting the apron.