Force feeding should only be done if ALL other options have failed. Even then, sometimes it's just better to let them go peacefully to their reward.
Personally, I've never found force feeding to be all that effective, but sometimes it works.
What kind of cage? (sorry if this was covered already) If an aquarium, try covering all of the glass so that she is completely isolated.
2 1/2 months is really not that bad. I've had several go more then half a year before they started feeding.