My son 3 year old son would only wee when he was sitting on the loo. It was because he was frightened of falling down into the loo. I reasured him by holding onto him and at the same time I got him to put both his hands onto the seat behind himself so that he could hold himself upright.
Thanks to all the people who replied to my problem. My daughter just decided a couple of weeks ago that she would now do it on the loo. I've decided that they just make their minds up one day to do it and no amount of coaxing and bribery will get you there until they decide..
Have you tried sitting your daughter the other way round on the loo so that she faces the cistern? She can then lean forwards and hold onto the cistern.
My daughter felt safer that way.