It is typically elev that causes the most problem with heading changes. Rudder can do it, but elevator seems to be the more common problem. It is usually a timing problem. Try doing the first half roll and stop. Check you heading. If you lost heading in that half roll think about what control input could cause that heading change. Then add the second half roll and check your heading. You should be able to figure out which control is causing your issue.

Woodie