Some photos are not devloping on my film camera

So I've had this problem which has happened with my last two rolls of film. And basically some photos aren't developing. They don't appear on the negatives, nothing. I really can't understand why nor have I found the answers online. I didn't know if it's a problem with my camera (canon ae1 program) or how I'm loading the film, something else I could be doing. One roll of film only developed 12 of the 36 and the othe roll I think developed 24. For my recent roll it was photo 1, 2, 14, 15, 16, 18, 19, 20, 21, 22, 31, 32, 33, 34, 35, 36, that did not develop. Also, I was wondering why some photos that I got developed have the number (for example if it was the 4th photo it would be called "4a" but maybe the 17th photo would be called "17" so there sometimes is an "a" there??)