Its been my observation that while doing a basic tenkan from same side grab is simple in principle, it is often difficult for new students to perform. I have an idea for a drill that may help. I have discussed it with some of the students at my dojo, but haven't showed it to beginers yet, and would like feedback.
Hold your arms nearly straight with palms together, or fingers interlaced. Bend your wrists away from the side that is grabbed, and down. Then perform your pivot with the wrist as the turning point. I hope this will hold the grabbed arm in a position approximating the proper form.