I've got 900 miles on the bike, I usually stay in the 5th cog in the back and middle ring in front on most rides, but down shift on hills. Recently I've noticed when shifting from 4th to 5th that the chain has a hard time grasping the cog. I've never deep cleaned the bike before, and assume the stone dust trails I ride are gunking up my drive train causing this issue.
The chain isn't loose by any means or has any indication of wear but I've noticed if I drop the middle ring to the lower ring and shift the rear into the 5th cog i don't have a problem shifting so its definitely the 2 x 5 combo. The mechanic recommended using a can of White Lightning degreaser to clean up the derailleur and cogs, re-lube the chain, and see if that corrects the issue. If it doesn't I'll be taking the bike to him to look at.