My advice to the original poster is this,
If your tuck is not rotating fast enough, it is mostly dependent on how fast (or if you're having rotational issues, slow) your knees are being brought up into a tuck.
A back tuck operates on the basis that after you set, you also pull your head back (note, NOT tilt your head up, these are different principles) which sends your body into a tilt, like this slash \ <- your body in mid air.
When your knees come up the momentum slingshots your weight around - you open with your legs to stop/stall/slow the rotation and to land after rotating enough.
So with this in mind, I think it's probably something to do with...
1) How high are your knees going. Are they actually going into a full on tuck? Alot of my students tell me "Yeah I'm tucked!" but they're clearly doing their back "tuck" with their legs out in a pike with bent knees - we have to record them until they go :O "Oh."
2) If you are going into a full tuck and having rotational issues, it could very well be how quickly are you tucking. A backtuck isn't really like a "tuck jump" its a tuck open, a Tuck open is a front and back tuck without the rotation - theres an open in both.
Ask your coach if you can't tell if its one of these things - ask them "Am I not tucking tight enough/fast enough".
@SHADOW
Hello. Back tucks are fun to teach.
I spot them on trampoline - however before I do this, I have them do a backward roll off of an elevated stack of mats about a spotting block turned on its side height (depends on the size of the athlete)
After trampoline, tumbletrack, then from two panel mats onto an 8" mat. Then snapdown into a backtuck, then round off, then ro-bhs-back tuck.
I hate teaching front tucks. However, this is how I do it. Have your kids at one end of the trampoline, and pull an eight inch meight onto one side of it, so only the side they're standing on isn't covered. Have them do a front tuck to a pike sit, practicing their arm motions. Then off of the tumble track with a run have them do the same exact thing up to a stack of 8" mats (maybe four or five high) and have them land in a pike as close to the edge as possible. Keep reminding them that front tucks go up, not forward. Explain to your kids if they're old enough to understand, if they can land in a pike on top of the stack, if you took the mats away - their legs would be falling down and catching them, they would be landing front tucks.
Then springboard->8" mat on floor, that's at least how I do it, there's so many ways of teaching those two skills though its ridiculous
Good luck you two