Definitely sounds like a DMX compliance issue on the fixtures. FLX S has two outputs - a 3pin and a 5pin. If you have a 1 Universe FLX S, by default both ports will output the same DMX data, but the 5pin will include RDM and the 3pin won't include RDM. This behavior can be changed in SETUP > Inputs & Outputs. Another useful trick is addressing the fixture into the higher numbers. Usually above address 32 helps, but if not above address 257 should definitely help.