I can't find the Derby X on the ADJ site, either as a current or discontinued item. I did find it as a Chauvet fixture.
First, does the fixture have a DMX status/presence indicator light? If so, they typically blink, some very rapidly(like my Chauvet Intimidator Colors) or are on solid like on my ADJ Dimmers and power packs.
There have been some reports of problems with Chauvet fixtures and MyDMX. All I can say is that I don't have any issues with my Intimidator Color 1.0's, my 2 Follow Spot's and my 2 Q-Spot 150's. It is suspected that the MyDMX dongle doesn't put out a full strength signal, which my account for the issues you are seeing.
DO you have something like the OptoBranch/4 to front-end your connection? I use this between MyDMX and my various runs for lighting and I have yet to have any issues.
In test environments(MyDMX dongle right to the fixture), I also had no problems. I never tried going to my Chauvet fixtures long distance without the OptoBranch/4.
Neat fixture. Contact Chauvet. Just a warning that my experiences have been less than stellar with that company. I'd suggest calling over email, but I always suggest going that way anyways.