There are two areas of concern here:
Your scope, as mounted, will have a high axis over the bore. At the ranges you are shooting the angle between the bore and line of sight will result in a significant departure from the distance you zero and any other range. Gravity will have no bearing at 10-20 yards. Parallax will have minimal effect at that distance.
The second problem is your group size. Something is seriously wrong. Either the scope is defective, the mounting system isn't properly tightened down or (no offense intended) you're a rotten shot. You did state that groups are better with the iron sights. If you're shooting 1/2" - 3/4" groups at 20 yards with iron sights then the scope and/or mount is the problem.
Your scope, as mounted, will have a high axis over the bore. At the ranges you are shooting the angle between the bore and line of sight will result in a significant departure from the distance you zero and any other range. Gravity will have no bearing at 10-20 yards. Parallax will have minimal effect at that distance.
The second problem is your group size. Something is seriously wrong. Either the scope is defective, the mounting system isn't properly tightened down or (no offense intended) you're a rotten shot. You did state that groups are better with the iron sights. If you're shooting 1/2" - 3/4" groups at 20 yards with iron sights then the scope and/or mount is the problem.