Reset the dwell back to stock if it has been reduced. In fact, if it is very cold where you are, you might even want to try increasing the dwell beyond stock.
Speaking of cold, this will effect batteries, so make sure that even if your battery is fresh, that you are using a quality brand. Cheap no-name batteries often don't perform as well as name brands, so if they are underperforming in the first place, being cold will do them no favours at all.
Make sure that the back check ball is clean, round and undamaged, and that the ball is no smaller than 4.00mm is diameter. 4.00-4.10mm is optimal, but a little larger is fine.
If the vent in the divot behind the can o-rings is any larger than a pinhole (it should be 0.3mm is diameter), that could rob pressure from the firing chamber when the solenoid cycles to fire the marker.