As a Seguin owner, I feel you. He is on pace for 299 shots this year. His career SH% is 9.9% If he were scoring at an average rate he would be about a 30 goal guy. So the shots are there. The goals, however, aren't. 11 goals on 204 shots (5.4%) is underwhelming.
This is year 1 of a $78M, 8-year contract. Is the pressure getting to him? Is he mailing it in? Is he too comfortable to get to the dirty areas that made him successful in the past? Does he have an injured hand/shoulder/upper body? Did he get a new girlfriend? Death in the family? Traumatic event? Is it just dumb luck?
It is hard, or impossible to know the answer to these questions.
I think what you can look at is TOI and shot volume. Seguin's TOI has been declining slightly year by year, but he is still over 19 minutes a game. His PP TOI is down 20 seconds/game than last year, 30 seconds/game since the year before and about 60 seconds/game from 2013-15.
Seguin's shots per game is hovering close to his career average.
Is Seguin a 40 goal scorer? No. That was a career-high.
Is Seguin a 30 goal scorer? Likely. That looks like about where the numbers put him.
Is Seguin a 25 goal scorer? More likely, especially this season.
Seguin's dropping TOI and PPTOI (as the team around him improves) look like they should shave some offensive numbers from his stat-line. His shot volume looks pretty consistent. If Dallas could draw more penalties and have a better powerplay (both are now middle of the pack) there is a chance at an uptick.
If I had to guess... I'd say that (in his current situation) Seguin is 28 goal - 42 assist player moving forward, with the odd year he has a spike in production.