I was having the same problem, and updating to the newest version didn't solve it. What did work was following the Nuance customer service suggestion of creating a new profile and doing all three voice trainings at once. It took about 20 min. altogether, and now I think the software is recognizing my words better than ever. (Then again part of the reason I wanted to do the additional voice training was because I had tweaked some settings that had made it less accurate than when I first installed.)
Note: to create the new profile, I dragged the old profile into the trash, and started from scratch. I initially tried to just overwrite the original profile, but that caused the software to crash.
The only thing I haven't been able to teach Dragon is to correctly spell my own name…