I'm 55, so it's been a LONG time since I've nursed...but I don't think much has changed over the years.
As I recall, your milk production will increase the more often you nurse (or pump), since frequency of nursing sends a signal to your body to increase the supply. I nursed my son during a period of time when I was very ill (nothing contagious or harmful to him). I was barely eating much of anything and, when I did, I threw up...but I had no issues with my milk supply provided I nursed him often.
So, perhaps part of the problem is you're only pumping once a day. Would it be possible for you to pump more often and/or actually nurse the baby during the times you're home? That way, you'd send a signal to your body that more milk is needed. Just a thought.