Have you guys had problems with dimmers on motion detectors (dimmer in the load path). The lights don't even glow. I can make the dimmer work and I can make the motion detector work but not together. I have tried 4 different types of dimmer and 2 motion detectors. The MD is turning on and I see 120 at the dimmer, it just does not put out. I swapped in and out on the dimmer. Still no joy. They are light bulbs Tom Edison would recognize so that isn't it.
I'm guessing your motion detector has a solid state relay type output. Both devices are probably designed to work with a sinusoidal input and a resistive load. The MD won't "see" a load when it tries to switch on, and /or the dimmer won't see an appropriate supply to trigger its electronics.
Perhaps a motion detector with an old fashioned mechanical relay would solve your problem.
I've not tried your setup, but the incompatibility of some dimmers with some led or cfl lights is well known and similar