Horse won't go forward?

hey, ok,so I'm exercising a 14.2 highland cross pony around 13yrs old. I went out on her without any other horses and the owner walked beside me,but whenever the owner stopped, the horse(sugar) wouldn't walk any further. I'd talk to her nicely or use my legs firmly,nothing worked she wouldn't go forward at all, in any direction. then the owner would walk forward again and Sugar would go forward. the week before, they brought another pony out to go infront and Sugar was great. Do you have any advice? why is she doing this? how can i solve the problem? thank you in advance.