It could be that something is affecting the way the medication is working for you… a change is physical or mental health, a change in diet, less exercise, etc. A healthy lifestyle amplifies the efficacy of medication and any other treatment.
There are other reviews of Vyvanse here and you may find others who have experienced the same: https://www.additudemag.com/medication/vyvanse/
Penny
ADDitude Community Moderator, Parenting ADHD Coach & Author, Mom to teen w/ ADHD, LDs, and autism