Are you telling it that you want to use the rear feed? In the print dialog box somewhere (I'm not at home to check right now), there is a box to choose which feed source to use. You need to change "Sheet" to "Manual - Rear" (if I'm remembering it correctly). You need to do that before you stuff the paper in, if you aren't doing that already.
I find that when I put the paper in the rear feed, it slides down a ways without much resistance, then there's a moderate amount of resistance as it continues in among the rollers, before it finally stops. Are you pushing it far enough to hit the "moderate amount of resistance" region?
Lisa