Hi there,
My DD half crawled, half commando crawled for ages where one leg would be fine and the other would drag out behind her. She has a physio work with her because she has lax muscle tone so that it was difficult for her to pull her legs together. We tried a boob tube like thing to hold her legs together (but I felt mean and didn't put it on her much). Anyway one day she just started crawling properly!
I don't know if your bub would have anything like this wrong - but you could always check with your MCHN or a physio or chiro to make sure there is nothing physically inhibiting her. It may just be a learning thing for her. Has she been doing it long?