My bathtub is not filling up. I tried taking out stopper and making sure it is in working condition. It seems like it is not sitting properly in the stopper area (even though I tried manually pushing it down) It still leaks water.. . could the stopper be different size than necessary ? Thanks in advance for any help provided.

Your plunger isn't set low enough, Adjust it lower with the adjusting nuts, (see image) Good luck, GTom