Happens all the time with rapid fire shifters of all the groups I've tried. What is happening is the factory grease is getting too thick to allow the rachet mechanism in the shifter to engage. I have found that squirting a solvent/lube like WD-40 in the shifter pod will help. The problem with this approach is that you are essentially breaking down and thinning out the lubricant, and that could cause problems down the road. This is where a set of thumb shifters really kick butt.