Hello,
You have have already tried this, but if you haven't, mix the other kinds of food you want to introduce with the banana. I went with one spoonful at a time into her apple mash (she really likes apples), until you hit a point where the new food versus banana (in your case) is too much, then they may not want. Then the next time, start at a higher new food versus banana mix. The heinz biscotti also helped us I think. We started with the apple biscotti, then when she liked the biscotti, we moved to other flavours. There is also a banana biscotti.
Let me try.. I found these few days she likes to eat runny porridge too. Mixed with chicken or pork. If beef, she throw it out also. Haiyoh..