John Collins needs the offensive system designed around him. Atlanta's unwillingness to do that is causing most of his inconsistencies. If Collins is able to move to a team that would prioritize his game (and he should try to force his way out), he will blow up as a player.
What Collins needs is to be the sole interior big man. He needs to be paired with a stretch five like Giannis is in Milwaukee next to Brook Lopez. That will free up the paint and allow Collins to attack the paint more both in PnRs & post ups where Collins is terrific.
Pairing Collins instead with a paint only big man like Capela forces Collins to spend more time (an inordinate amount of time) on the perimeter where he plays to his secondary offensive talents (outside shooting) instead of his primary talents (paint scoring). This leads to less scoring opportunities, lower efficiency scoring opportunities (long jump shots) and a decrease in high percentage easy paint scoring opportunities. All of which lead to increased levels of offensive inconsistency.
Easy problem to fix.