Is she by chance pleasure bred? Only reason I ask is because it's pretty common to see that in both english and western pleasure classes. Those horses are bred for such slow legs and flat movement that they really don't pick their feet up very much at all. I would suggest working her more often over poles so that she can build up the muscles that are needed to get some action in her legs.
Although, IMHO, the best thing for a horse like that is to be ridden out on the trails through rough terrain and deadfalls. That helps them to figure out exactly where their feet are all the time and helps them learn to pick them up.