CW2 Mission 20: Laser has no effect?

[evnafets]

I have had an awesome weekend working through the CW2 missions (only played the free internet version before) There were some nice challenges along the way.

I'm onto the final mission... and I'm stuck. 
The Dark Laser doesn't seem to do anything, with a mirror or without.

It doesn't kill enemy drones or creep, and doesn't seem to do anything to the nexus, even with all creep on the map obliterated.
From You Tube video I have seen posted the laser should mow through drones like a hot knife through butter.
The only way I have seen a laser damage a drone is when the drone walks onto it (after shrugging off the effects of the laser the whole way).  And the drone walked away from that encounter.

Haven't seen this issue posted anywhere else, but please point me to the right place if I've missed it.

I have had stability problems with the game (Ctrl+C, Ctrl+V was my solution).  Have just tried rolling back Adobe Air to version 19 as suggested in another thread.  Have to play a bit longer to see if that solves stability problems, but I have confirmed it doesn't 'fix' the laser.

Running on:
Windows 7 Home Premium edition SP1
Intel i5
8GB Ram
Graphics card: Sapphire Radeon HD5770

[Builder17]

First , you should make sure laser is armed. Is laser blue or red?
Second question , check this: https://knucklecracker.com/forums/index.php?topic=19732.0

[evnafets]

Armed?  The lasers are blue in their puny little ineffectiveness.
And oh!  I never noticed this big red button before....

ZAPPO!

(I feel kinda stupid now)

Thanks!

Edit: Oh, and yes the stability problems seem to be a thing of the past.  Or at least no crashes/go slows tonight.