Ponies are smart and quickly become very good at outsmarting people. Repetitive tapping, not on the rump, but behind your leg where your arm hangs straight down should elicit a response. Sometimes they will pop forward for the first stride so be very careful to not accidently pull on her mouth. If you tap her on the rump this seems to elicit a buck which you don't want. By tapping her, think of an annoying kid brother tapping your arm for attention and won't let you ignore it. He gets your attention, right? It doesn't hurt, just annoys and that's what you want to do with the pony. Be sure to stop tapping the moment she's moving forward. Be patient and don't quit until she moves or you've reinforced her behaviour. It might take a minute or ten minutes or more.