This probably goes for any other brand too, but if the "dot" seems screwed up out of the blue try blowing/brushing the emitter area to get crud out.

I lost most of the circle hash line from 12 to 3 o'clock yesterday and after verifying the lens wasn't smudged and my eyes weren't shot, I tried blowing in to the emitter area which returned most of the reticle brightness in that segment before I got around to brushing it which returned everything to normal.