Are you having her take off her panties and pants completely? She may need to totally remove them in order to be able to open her legs wide enough to straddle.
Also, do you feel like she is just being careless. With little boys, who miss because they are in a hurry, I always suggest having them help wipe off the seat after they are done. Not as a punishment, but as a responsibility. When you make a mess, you need to clean it up. Show her how to grab a few extra squares of toilet paper and wipe around the lid then throw it in before she flushes. Usually that chore helps even the youngest child aim better.