[souffle]

I have also found, like Vectis, that moving these nervous hamsters to a smaller, tank type cage while taming does help. Something like a mini duna, just for a couple of weeks works wonders.
It may be that she has had a bad experience that her owners have not told you about such as the kids poking things in at her or the cage being knocked over. It would also be an idea to give her a good health check as hamsters that experience pain when being handled will be reluctant to come out.
I'm sure you will manage to win her over with time and patience however she may not be the same when returned to her former home if she has indeed encountered something there to make her this way.
Good luck with the taming and gaining her trust.