My total guess is that Quick is still the slow learner he always was and because he missed time with the shoulder injury, he's behind in learning the new offense. His shoulder is probably fine, but as with the previous offense, he's just not getting it right away.
Fisher's not giving up on him because they KNOW what a physical talent he is, but... he's a repetition guy. I dunno if it's a learning disability thing or a learning modality thing or if he just has a low football IQ thing, but... much like old florescent bulbs, it just takes awhile for the lights to come on with Quick.
I still have faith in him, but I think that's why they didn't rush him back out there.