I'm not sure on your particular situation, but I've pumped for 9 months full time now. Pumping 8 times a day, with once overnight is recommended. I go for about 20 mins doing double pumping so equivalent to 20 mins a side. You might already be doing this. I actually found it hard to maintain a supply over the long term without attaching my bub so I'm now on motilium and my supply is great - but this sounds different to your situation.

Wishing you all the best and I hope everything works out for you.