- You want to be good in leg skill (such as foot sweep).
- Since your "internal" teacher doesn't train that, he doesn't know how to help you.
- You go to another external teacher who can help you.
- You lose confidence in your "internal" teacher.
If I liked throwing and sweeps I would probably choose to practice Shuaijiao instead. But I do have full confidence in internal arts and have no interest in this kind of sweep or similar methods.
All teachers are specialized, have their own limitations and so. And students should find their own talents and interests within the martial arts IMO. Some people find their own strengths in the external arts, others find it in the internal arts. One thing is just as good as the other as long as you follow your heart and find joy in what you do. My own humble opinion only.