I'm assuming that by 'no control' even the grand master is not controlling it?
If so, its unlikely to be the desk so the first thing is to find out of you are running analogue or DMX? Also are the lamps DMX or driven from a dimmer pack?
If analogue the first thing to check out is the control cables and connectors making certain you have no shorts between circuits or screen of the connectors.
If the devices are attached to a dimmer, that could also be the cause of the problem.
However, if grand master does affect the channels in question, then it may be dodgy faders. If you contact Keith Rogers at Zero88 he will be able to advise on spares.
If you can't resolve it, please post back with more detail of the packs, lamps and answers to the above so that we can help tie the problem down.
Robin