I had a lot of trouble w/back tucks. I had a good set, but I wouldn't rotate. My coach had us work standing back tucks off of mats, and I improved a lot. BTW I have my round-off backhandspring back tuck now.
Congratz on getting ur round-off backhandspring back tuck we call a back tuck a tuck back and a back handspring a flic where i come from, so i make all this translation a bit funny. Thank god for google.
For helping with your rotation of the back tuck try having your coach spot you on a few and only have them push your butt over your head. Also try doing some back 1 and 1/4s (do a buck tuck then an extra quarter of a flip so you land on your back)--this might help you understand roation.
In learning a BHS-tuck, I find that a back tuck on trampoline is FAR more useful than a standing back tuck. I have my girls do straight jump, straight jump, back tuck, with their arms never leaving their ears until the tuck (ie no arm swing). This teaches the proper pull with the shoulders, and makes it much easier for the kid to mentally isolate the set from the tuck.