If you have the ability to hunt with a good tree dog, do it. I have never really put all that much effort into building the want/desire to tree. It just seems like they do it or they don't. If you get one treed, tie her to the bottom of the tree. Pet the other dog real good as it trees, pet yours when she trees. Only when she is treeing. I wouldn't do it for a long time, 20 minutes. Some dogs will even shut down faster than that while learning. I have been lucky to have good locate and tree dogs to help the new dogs along. I agree with Jeff that at the dogs age it may never make a great tree dog. That doesn't mean she can't be useful or even great at other things. I will work my young dogs for around twenty minutes or so at most every tree. Only show the dog attention while it is treeing. Repetition, and she will most likely catch on. If she doesn't find a dog that will. Not saying you should get rid of her, just find her some help in the treeing department.
Probably not that useful of info. Just what I do. I try not to force it.