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