1. Weight change, either bulking on muscle or fat can change your optimal dose for medication... And it's my opinion (and totally just my opinion) that starting to treat ADHD usually leads to a weight gain, either by getting you more able to exercise or increasing your activity and caloric intake.
2. Resistances, stimulants build up resistances pretty quickly for most people so if you're unlucky you'll likely be med hopping for several years until you run out of options or find a winner - if you're lucky and don't build up a resistance then you're probably set for life.
Both of these are addressable though, in the first path any significant weight changes should be monitored by your doctor (go to your doctor regularly, blood pressure should be monitored while on stimulants) and ideally the medication adjusted up... for the latter, well, it's a pile of poo but if you're able to stick with it your doctor will likely take you on a tour of different methyl*s until you find one that works - then, try your damnedest to never let your insurance force you off of it.