This is not unusual. I was working when my older son was that age and would find that by the end of the week, my milk supply had dropped quite a bit. Nursing extra on the weekends and at nite will increase it best. If you're no longer nursing at nite, you might actually want to reconsider, as that's the most effective way to increase your milk supply AND for many moms that are away during the day, it's totally natural in that basically your nursing schedule can be flipped around, so you nurse when you're there.
Also, some of us have one breast that a baby prefers and thus that breast will gradually produce more and the baby has a preference. I had the same problem with my left breast - my older son would nurse very little on that side. Now, this might have been due to my having cystic breasts, which may have interfered with the milk production.
You also CANNOT judge your milk supply based on how much you can pump. I could NEVER pump more than 3 ounces at a time. Some of us just don't connect to that pump like we do the baby and thus the milk does not "letdown".
The most important thing is to not get too stressed - stress very negatively impacts the milk supply, which might be another of your problems with your busy schedule.
I would just keep working on it. You are very close to being able to add some solids. What I did was to reserve solids for when I was away ONLY. And, then we strictly breastfed at home until my sons were almost 9 mos old.
Also, you do say you're nursing as often as possible, but you don't mention any frequencies. The range of "possible" is very, very broad. I nursed before every single transition point in my day to keep up my milk supply when I went back to work (i.e., as soon as I got up, right before I left the house, right when I got home, right before dinner, right after dinner, right before bedtime, etc.). I know this can be difficult, but you can read to your daughter while you're nursing and read school work while you're nursing, etc. When my son was 18 mos. old, I knew he wanted to nurse when he brought me a book to read ;)