The left hand B lever (small lever to shift to small ring) all of a sudden quite working. It just moves freely without engaging. Stuck on large ring. Anyone familiar with this problem and how to fix it? Thanks.
Flood the insides of both shifters with WD40. This will loosen the hardened factory grease.
While I'm a believer in using WD-40 for flushing out both road and mountain Shimano shifters that have the dreaded "dead shifter syndrome" caused by old grease, I wouldn't advise it with 6800 shifters. Couldn't be more than a couple of years old, it's a pretty new group. There's almost certainly something else going on here.
Push the big lever (shift up) gently and see if there is still movement in the shifter/FD. Then try pulling down on the derailleur cable (slowly - don't tug) on the downtube (left side of the bike going to the FD) towards the BB while pressing the inner lever. This should help move the gear mechanism inside the shifter.
If the lever doesn't release with this then I'm afraid the pawl release mechanism is busted.