There could be several possible reasons why you get a no disc error:
If each deck has it's own regulated supplies, one of them may be faulty.
The disc may not be sitting at a tangent to the laser. Screws sometimes slacken a little and this knocks off the tangential adjustment, which can play havoc with tracking and focus, although I don't think this is your issue, unless you have played with the screws.
The spindle motor might not be quite making it to speed, causing tracking issues and making it near impossible to read the TOC.
The lens motor might be jammed, or goosed, causing a focus servo issue.
The laser receiver might be goosed, in which case it's effectively "blind" and won't see or focus on a disc.
Out of the possible causes, I'm betting in this case that the laser is goosed, but I would check the supplies first.
Try swapping the faulty laser and mech complete onto the good channel, if it plays OK, then you know that there is some other problem, if it doesn't play OK then I would just buy a replacement mech complete. It's only a couple of quid more, and in some cases is cheaper than a replacement laser on it's own, has all new motors and comes setup and ready to drop in