Is it possible that you are getting different generics? There can be a pretty big difference in the efficacy of generics (like up to 25 or 30%, I think). Seems unlikely that the same manufacturer is changing formulation, but pharmacies change the brands they stock pretty frequently.
You can ask the pharmacy to stock a different generic for you. We’ve done that before and most don’t mind doing it.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism