So, some times, shifting between the middle and large chain ring, my lower thumb shifter goes blank, and after a few back and forth movements, it will sometimes engage. What could be the problems, dirty housing, something within the pod? I noticed with my ultegra shifter (right side) will do this from time to time on my road bike, go to shift and nothing there, then eventually it will engage.
Need to clean and lube the shifter. To clean use something like white lightining clean streak followed by som isoprop alchol and blow out with air and let dry and then lube with a silicon lube. SHould do the trick