I recently had a new system put in my toilet with a flapper. The toilet kept emptying a little and then would fill up at intervals during the day. I told my plumber about it and he came and replaced the flapper. We're on the second flapper and the toilet is still running. It seems the flapper isn't seating right. We already tried the vaseline. Are there different sizes for flappers?

It's just annoying and I'd like to fix it myself without having the plumber come back. Thanks for any help you can give me.