There's no simple answer. Sometimes he's hurt. Sometimes it's matchups. Sometimes his partner is on the fritz.
Discounting a simple "he sucks in the playoffs", Cassidy's Bruins had trouble scoring 5v5 in the regular season and a real bitch of a time getting it done in the playoffs. And Grzelcyk's TOI is petty much all even strength.
Monty's got these guys firing much better so far. They'll probably top last season's 5v5 goals scored next week. We'll see if the Bruins even strength scoring is going to raise Grzelcyk's ship this spring. If it doesn't, a lot of historically batshit numbers vs reality conjecture will take a big step in pointing out a trend.