Usually the problem is with Interlock. Some lasers use pins 4 and 17 of DB-25 to detect, whether the external laser show controller is connected. If this is not connected inside ILDA cable (which is quite common, since you save some wires), there’s no way for laser to know, there is external control required. Easiest way is to short manually pins 4 and 17 on your ILDA cable.
But some of the lasers also need to set the DIP switch to certain position (e.g. 0) to be able to use external control. This is something you need to consult with your laser manufacturer.
Hope it helps.
Michal