sometimes ukes fall before you throw them because they are new, and want to have better control over their falls. Increasing your intensity if this is the case will only make them more likely to fall early.
Or she may just never come around to the usual dojo way of things.
It may also be that you are incorrect in your assesment of her ukemi: if she is ahead of your technique, perhaps it is because her ukemi is good enough to stay connected to you and move to maintain her integrity. Or you may be larger and over muscling the technique, and she is making sure she stays enough ahead that you don't break her. SOme folks want an uke that resists with all their strength, even past the point that any intelligent person would take the fall to regain advantage. If you are bigger than your uke, they are just not going to be able to give you that kind of resisitance, never mind that it is stupid to do so anyway.
Or, she's "just not doing it right." When I have a scared uke that falls early, what I get out of working with them is figuring out how to reassure them (smooth, gentle technique) so they don't feel the need to rush to the fall. You might try saying "it would help me---note: not 'you're doing your ukemi wrong'--it would help ME if you could not take the fall quite yet, I'm working on the part just before and want to focus on it." If she says she can't help it, she's already lost her balance, then hooray for you, you are actually throwing her and perhaps missing it because she's so much smaller it seems too easy. Or she may try to help you by staying upright a bit longer.