When pedalling in reverse, there's nothing that acts as a proper guide to the chain any more. The more cross-chained(big-big/small-small) you are, the bigger the risk of the chain derailing.

If normal shifting is fine, then don't expect gear adjustment to sort it out.

But why back pedal? Anything more than a half turn at most (a quarter turn would usually be sufficient) kinda indicates poor pedalling technique.