I've done this a million times, but this time I cannot figure it out.
No 3D layer in my comp will accept a luma matte. What could possibly be the reason for this?
If it is 2D, it accepts the matte fine, but the second it switches to 3D, it loses the matte.
Would really like this to work so I don't have to rotoscope everything.
What renderer, Ray-Traced 3D or Classic 3D?
You probably have Ray-traced rendering turned on. A warning should have popped up to tell you that the effects and track mattes will not work on 3-D layers with this rendering engine turned on. You may have turned off that warning the first time you saw it.