Adderall can be a fickle ADHD medication for a variety of reasons, but one may be related to the nature and quantity of food that you've eaten around the time you take it.  The reason for this is due to its delivery method.  Other ADHD medications such as Concerta are encased in a way that they are delivered in a more "osmosis-controled" manner.

This form of delivery makes them less sensitive to food you've recently eaten.  Adderall, however, exists in a more "beaded" form and is quite a bit more susceptible to the surrounding environment.  I just read a study that was done a few years back that measured the changes in absorption between Adderall metabolism on an empty stomach vs. Adderall metabolism after a fatty meal.  The difference in absorption and net effect on ADHD were noticeable-- the overall effectiveness was reduced after eating the richer foods.  Since you're on a relatively low dose and are taking it twice a day, the "foodeffect" might have an even greater relative effect on you (I think the study was done on a slightly higher dosage, if I remember correctly). 

If you're going to stay on Adderall, and notice a decreased effect, double check on any recent dietary changes.  I could be way off, but your food choices may have a greater effect on the effectiveness of this ADHD medication than you think!