Different manufacturers can make a huge difference. The generic can be as much as 15% different from the brand (I think that number is correct), and be approved as a generic. Plus, the additives can be different, as well as the time-release mechanism. That can add up to a huge difference.
If there’s a specific generic that you did well with, call around to different pharmacies until you find one willing to stock it for you.
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism