Never having been in a physical fight, or physically attacked someone before starting Aikido, I had to learn how to attack. My attacks were pathetic at best. I also learned the stronger Uke attacks, the faster they come out of the technique done to them. I've only been training a year and for the most of that time been a very compliant Uke. I was respectful (fearful) of the techniques (not of my Nage(s) but of the potential of what a technique could do to you). Not understanding the dynamics of the movements, I felt more comfortable never resisting for fear I would do so at the absolute wrong time and really get hurt. A year later, after doing certain moves again and again, I've begun to feel more comfortable about providing some resistance and providing my Nage with a more realistic attack.
Some folks are just more hesitant, not willing to be sincere (aggressive) in their attacks or ukemi. Colleen's advice above is exactly what helped me get over the incredibly uncomfortable feeling of not being in control of falling. Nage's at our dojo go real slow with new folks so they, as uke, can wait until they are thrown and still set up for taking that backward or forward roll. Since this gal has only been there a month, regardless of the fact that she came from a different style, please just consider her new and don't expect just because you've explained and demonstrated means she feels comfortable changing how she does things only after a month. Wait to see how she is doing after 6 months, or after a year.