For Ann Arbor residents and University of Michigan students, filling a prescription for attention-deficit/hyperactivity disorder medication has long been an exercise in uncertainty. Even before the Oct. 28 recall of lisdexamfetamine dimesylate capsules, commonly known as Vyvanse, pharmacies in Ann Arbor and across the country have struggled for more than two years to keep key stimulant medications — including Adderall, Ritalin — consistently in stock.