I guess I read the OP the opposite way - that her knee is fully extended at the bottom of the pedal stroke and she really has to reach for it. Then at the top of the pedal stroke, her knee is excessively flexed, so her foot slides forward on the pedal.

I vote for crankarms too long ... but I'm biased because it's such an issue for me.

SoInTents87 - is your issue that your knee is not extending enough, or too much??