Does the ADA cover this?

I've been working at Roche Brothers as a service clerk (euphemism for bag boy) for almost three years. I've asked several times to be trained as a cashier but they deny me every time saying that I have to "master" bagging before I can become a cashier. I can't "master" bagging because of my severe ADHD. It requires the two things that I don't have; a fast processing speed and organizational skills. It's incredibly difficult for me to do my job efficiently, especially when there is so much freedom to walk around. It's too easy to get distracted, particularly when it gets slow. I'd be much better as a cashier. The only organization required is handling money; everything else is very straight forward. Most of it is memorizing codes, which I'm good at. Cashiers make the same pay and aren't even technically a "promotion." Many employees start out as cashiers rather than baggers. They're equal. So my question is: is it legal for them to deny me the training? As far as I know, diagnosed ADHD is considered a disability by ADA. And I'm fairly sure this counts as discrimination based on disability; they won't train me because they don't think I'd be able to stay focused, work efficiently, etc. Oh, and my boss is fully aware of my ADHD.