1) this is general advice to be taken with a grain of salt because I cannot see your dismount so I can't diagnose the problem
20 90% of the time the root issue I see on this dismount is gymnasts directing their momentum down or out instead of up-. You can fix this by using your arms to set up, and you can set up by finding a slightly-above-eye-level fixed point on the wall in front of you, and look at it when you punch off the end of the beam.
Other issues can be lack of speed during the rotation (so take more steps into it and be more deliberate in your rotation), or a weak punch off the end of the beam that absorbs your power rather than adding too it (so don't bend your knees when you punch, stay stiff). Both of these are dynamic issues.
I would observe your dismount and see if either of these is the problem, or if it is a combination of these.