Did you get in on the DMX Tester coop? This kind of problem is one of the reasons that device was created. It would be a very useful tool to help you debug and find out where the problem is (LE, communications, computer/vixen, etc). If you didn't get in on this coop, make sure you get in on the next one.
If I had a second one built, I'd loan it to you. I do have a few extra PCBs if you want to build one (if you weren't in on the coop).
Dave