Who knows, maybe Dennison is incompetent. It's quite possible.
However, let's say for sake of discussion that he is not. Why would the offense be having such problems after such efficient seasons in 2015 and 2016 if Dennison is in fact good?
Offensive Line - Cordy Glenn's injuries, John Miller's regression, Jordan Mills is still Jordan Mills, and the overall learning curve of a new and entirely different blocking scheme has caused the OLine to go from very good to not-so-good.
Taylor staying in the pocket - Considering the absence of the read-option play and Taylor remaining in the pocket like a prototypical NFL quarterback, it's been much easier for opposing defenses to stop the run. They don't have to take Taylor's mobility into account nearly as often. You could say, "Then why doesn't Dennison put in the read-option and let Tyrod run more?!" And I would say it's a call the entire organization made before the season began: seeing if Taylor could be a real NFL quarterback who could pick opposing defenses apart with his arm.
Wide Receiver unit - A regression in the skill and talent from this unit considering injuries and other factors. Without having to pay quite as much attention to the WR's, opposing defenses can stack the line and dare Taylor to beat them with his arm.
Tyrod Taylor is still a flawed NFL QB - Taylor still has trouble making his progressions at a sufficient rate, still lacks pocket awareness, and still refuses to attempt to throw receivers open on a consistent basis. Whether the wide receivers can make the play or not - it's Taylor's responsibility to attempt the passes instead of hesitating and taking the sack. Because of this, he hasn't been able to keep defenses honest and prevent them from stacking the line.
Couldn't all four of these things be true? And if so, wouldn't that result in the offense's regression even with a talented offensive coordinator?