I have the, kinda, the same problem with my generic Adderall (Barr/Teva Manufacturer). I was taking 10mg twice a day for about a year and ever had a problem with the effectiveness. I was increased to 15mg twice a day and noticed that it does not work nearly as well...if at all. There are times I wonder if it is a placebo pill or if there is some problem with the amounts in each pill during manufacturering. Sometimes I can tell very little that I took it and other times, it feels like I took nothing at all. I also hate the fact that I feel very tired on the 15mg...almost like I have a hangover most of the time. I went back to the 10mg for a few months and they were effective but not enough of a dose after taking it for a little over a year. So, thinking maybe it was me, I asked the doc to try the 15mg again, which I am taking right now. I HATE IT!!!!!! It doesn't work and I've tried finding another generic brand, such as Sandoze (not sure if spelled right) but could not find it at any of the pharmacies near me and the brand name was on back order.

Well after giving the 15mg a second try, I am going to try to maybe ask to have some combo of 10mg ie: 10mg 3 X/day, or 10mg 2 tabs in am and 1 tab in afternoon
I'm not sure how my insurance will feel about that but I know that the 10mg works and the 15mg does not. The odd thing is that they are both from the same company. I am completely sure that there is a difference in the two just don't understand how the lesser dose works better and why. I plan to write the company...not saying that they care but no harm in voicing my concerns.

To answer your question, I have noticed a difference in effectiveness of dose and there may also be an issue with your 20mg. You may want to try for the brand name or a different generic. I would also ask for another dose in the afternoon if I were you. You could always try and go with the 10mg twice a day, which may work for you as I know it did for me.

Good luck!