I can't say for sure because I have not seen you dance. But my two suggestions won't lead you astray no matter what the root cause.
1. Practice. Keep working on the movement over and over and over. Hold onto something in you house and do it until it looks and feels right.
2. Work on your balance. Rumba is about balance. Balance is about strength in your core and your ankles. So you need to do some excercise that does that and keep doing the movements until you can do them by yourself and without anyone else supporting you. If you have an instructor who tells you its okay to be off balance and use him as support...don't listen. You need to be able to perform every movement on your own without wobbling and without help.
Those are my...at first blush suggestions.
Good luck!