Because of his limited minutes and newly defined role (long range specialist) Jimmer is in a tough spot. When he comes in, he has to be on or he's going to get yanked right away. Players in that position don't get the luxury of warming up or working through a dry spell. He's on the floor for one reason, to score from long range. And if he starts out slowly, he'll end up riding the pine the rest of the game.
Players like Harden can go for 0-12 in the first half and get the liberty to work through their off night. Not so with Jimmer.