I've noticed this too. I've been on 20mg XR twice daily for years. I took name brand Adderall until about 3 years ago. Usually it's fine but I occasionally run into a pill that seems like it's a placebo or just diluted. There are a few different makers of generics, one time my pharmacy had to give me mixed bottle of two different makers and I noticed some slight differences but I can't rightly recall the specifics. I figured my mind was playing tricks on my so I put it out of my head.

Quoting from wikipedia on generic drugs, "The FDA requires the bioequivalence of the generic product to be between 80% and 125% of that of the innovator product.".

Adderall is made of 4 different amphetamine salts. From what I've felt, it seems like the generics have slightly different ratios of those four salts. This is pure speculation, I really don't know for sure, but they feel slightly different in very subtle ways and that's a reasonable, though currently unsubstantiated, explanation.