Z axis belt fraying

I am a noobie as well but have been able to make quite a few projects and my XXL has been performing well until 2 weeks ago. In the middle of a project the depth started to change mid project. I checked set screws after reading another post and they were good so I looked at the belt and noticed fraying. I replaced the belt and thought I tightened the belt but not too much but on the very next project the depth got screwed up again and sure enough the belt has started to fray again. I looked over everything and see no reason it would fray except for high tension or a faulty belt which I think is doubtful. Any ideas?

Check to see if the belt tracks straight — examine the pulleys and idlers for any nicks or flaws which might damage the belt.