Pumping issues

I stay home with my 9 month old 5 days a week and breastfeed on those days. The 2 days I'm at work I pump every 3 hours during my 13 hour shifts (he eats about every 3 hours at home). These past 3 weeks I've been pumping significantly less. My average bottle was 4oz, then dropped to 3 and now today is 2oz (combined from both sides)! His checkup was this week and his growth is stable and he doesn't seem fussy/hungry when I breastfeed so I don't think it's a supply issue. I've been pumping at work for 5 months now and my production hasn't changed until these last 3 weeks.

What changed? How do I pump more? Should I start trying to up my supply even though he doesn't seem bothered?