Cohen is small so when you ask him to be one of your main weapons on offense its a mistake. In 2018 when you had robinson, gabriel stretching the field, miller, and burton with solid blocking he was very effective because he wasn't one of your main options defenses were paying attention to.
Not the same in 2019 with horrendous blocking when he's 5'9 and the only weapon on offense besides robinson and muller so the defense can focus on him. Him being your #3 option on offense is a recipe for disaster and thats before you add in the awful blocking last year.
Smaller guys like him need to get a bare minimum level of blocking to make an impact so when your line looks worse so do they, which he never had last year. Doesn't mean he isn't valuable, you have to create space for guys like him
When you improve your offensive line and weapons (and hopefully QB play one way or the other), he goes back to being a weapon and making explosive plays