..A little squirt of contact cleaner or WD40 on these switches and pressing and releasing them numerous times is usually all it takes to fix this problem... 
don't use WD40 as it's leaves a greasy residue and attracts alot of dust/debris...I've been using Jigaloo (orange can) for quite some time now & it's alot better as it leaves no greasy residue and it smells nicer
