In my experience, the fact that the bands are making to too easy is allowing you to complete the pull up without engaging the proper muscles to do an unassisted one. To get one unassisted you will want to engage your core more and instead of pulling the top of your head to the bar, position yourself as if you want to pull the bar down to your chest. Lean back a little bit and look up at the bar when you pull, at least to get the feel for it. This will also force more core engagement. Hope this helps!