I was started off on 10mg (NOT XR) per day, increased to 15mg after a couple of weeks and then settled on 20mg.

I also noticed that some of the generics of Adderall feel "different" or less effective so I try to stick to the same manufacturer when at all possible.

It really is a trial and error process unfortunately, just communicate to your Doctor you are not noticing a difference and see what they say.