Theory 1: Some guys just can't be on all the time. They're able to raise their games and do great things on occasion (enough to keep them in the lineup and give them extra rope from the coaching staff), but not with consistency. I'm thinking of Michael Ryder here. You'd look at him when he was on fire and think why isn't this MFer scoring 40 goals a year? Then he gets into a funk and looks like he's floating for 20 game stretches.
Theory 2: David Krejci made him look like a much more talented scorer in his first few years than he really is. Playing with Coyle he doesn't get as many perfect setups. Of course, he had plenty of time with Krejci where he did absolutely nothing and Krejci's had a lot of wingers like Seth Griffith and Brett Connolly who never produced half of what Jake did on that line.
Theory 3: The concussions and injuries from the Kadri hit and the Covid year clipped his development and he's never going to get any better than this.
As with anything, nothing is monocausal and it's probably a little bit of all the above plus other things. Could be a mental block, a physical injury, the way Bruce is deploying him, chemistry with lines, etc. At this point I see a guy who is half Michael Ryder and half Reilly Smith, I'm sure that if he stays with the Bruins he'll continue to be a 12 goal, 30 point 3rd line player that he's settled into, and when we trade him to Edmonton he'll put up a 29 goal, 55 point season and everybody will be all hOw CoUlD tHeY tRaDe HiM fOr A fOuRtH rOuNd PiCk?/??1?1?