Free Terran, maybe you forgot to enable glow on the scene? Using two planes like that is a very good idea for making beams, with post glow of course - I might steal it ^_^ Anyway, yeah, to enable glow in the scene, you need to make sure the surface of the object has glow turned up, then go to scene menu, select the 'more' under 'effects', and select image processing, then select 'enable glow'. Play with the settings for desired effect.
Oh, and all you people saying "why not do it in a 2D program", well, you try making something in a 2D program convincingly and consistently interact with the surroundings in an image. Like, if you want the beam to actually hit something accurately and to scale along its length. Or, if you want it to properly illuminate something. Basically, because it looks better when done well.