Come is the easiest and the hardest command to train. A couple of quick tips - first, only use the word "come" when you can enforce it. Start on a standard leash and make it fun! Certainly use a great treat - responding to the command should mean that wonderful things happen! As she comes reliably on a short lead (and please remember - only use the word "come" once) then start using a longer one.
The second tip is this - only use the word "come" once! If you keep repeating it, then the dog learns that listening to it is optional. This is one command that is NOT optional!
Just out of curiosity - have you had her hearing checked? Is it possible that maybe she doesn't hear well?