This is a speculative post, after finding that quite a few people tend to improve much more on amphetamine-based medicines than Methylphenidate.
I would assume that those who are non-responders to Methylphenidate have a deficit in the actual release of dopamine. From what I have read, these patients may lie more on the ADD/general lack of motivation side than the more hyperactive side.
This problem could be resulting from a mutation in the actual vesicular transport of dopamine, rather than the Dopamine Transporter itself. But for amphetamine or similar-based compounds to take effect, the dopamine transporter is required.
So I would assume in this subset of patients, a DRI like Methylphenidate would actually impair their remission.
Interestingly, nicotine, which is commonly used as self-medication for ADHD, has an up-regulating effect on DAT. But clearly this chemical has considerable adverse effects as well.
So my curiosity has been in compounds which have a normalising effect on DAT (that may enhance the efficacy of an amphetamine).
I have gathered that L-Arginine, and L-Carnosine have this capability.
It was also interesting to read that a combination of caffeine + Phenylethylamine (of which seems to be present in Cocoa in low amounts) to mimic certain responses to amphetamine, though I doubt this has clinical significance.
I agree with Psychiatrists in that amphetamines should be tried only after a trial with Methylphenidate has failed, as amphetamine can have much direr consequences on the nervous system if abused.