I've been on generic Adderall for three years and there are some days that it seems to work better than others. There are external factors which I know help me.
1 - Exercise: When I get my morning and evening walks I feel much better
2 - Sleep: If you don't get enough sleep the meds don't work as well
3 - I recently started taking Omega3 Fatty Acid pill and also Juice Plus, both recommended by Dr. Hollowell in "Delivered from Distraction"

The jury is still out on the supplements, but Exercise and good sleep are really important.