Sticky Di2 shifter
 

I have 3 year old 6800 series Ultegra Di2 on my bike and when I took it out on a ride this morning I noticed that the left shifter is sticky. It’s the actuator closest to my body which shifts the chain from the small chainring to the large. It will shift but not crisply and it will return back to the home position but again not crisply. The other actuator on the same shifter is flawless as is both buttons on the right shifter. I wasn’t able to look into it this morning, but it feels like there some gunk up there that’s muting the response. Can I try a few blasts of WD-40 to try to clean it out or is there something else I should use? Is it possible that there’s a spring that fell out?

Yep, each shifter contains two 'switch springs'. Have a look at the Shimano docs website (si.shimano.com) and then search by keyword: ST-6870, open the EV file (exploded view).
I would link to the file, but I can't link to files yet ;-)

(it is called EV-ST-6870-3642.pdf, look at the items labelled 5 and 6)

I tried some electrical contact cleaner and some WD-40 and neither helped. I'll check out the diagrams above and try to figure out my next move.

Just a follow-up..... a few days after my above post, the shifter is working as expected. I guess there must have been some gunk in there that finally dissolved away from either the contact cleaner or WD-40.