I have the same issues. There are some semesters in which I am very capable and in some I'm doing less than my potential. I think it's the different phases in our lives. We aren't always the same everyday. Change happens gradually and sometimes we aren't even aware of it. Could be that you are dealing with a different environment with different circumstances. New worries, a changed awareness, new priorities, different pressures... could all be contributing to the difference in how we are doing. It's important to be aware of ourselves.