we practiced come at home until our pup would stop whatever he was doing and come, he's now 13 months and i can now finally let him off the lead and be pretty much confident he will come back to me on command.
he also goes on an extendable lead and i practice come with that too when we are out.
My prob is that he's not interested in treats when we are out so he has loads of praise instead, see what your pups thing is and practice come giving the treat or whatever then letting her go again and then repeat it until she learns that coming back to you is much better.
puppy classes should help too