S.S.
I'm not sure what kind of pump you have, but it could be that some of the pump parts need to be replaced. Many companies that make breast pumps have a 1-800 number you can call and speak to a lactation consultant, or at least a customer service person who is knowledgeable on how their pumps should operate. They might be able to offer some suggestions. I did that for my Medela PIS Advanced when it wasn't working well, and it turns out there was a flaw in that model that made it need a new faceplate and tubing. The company replaced it for free! It also pays to replace the membranes regularly if you start to see a noticeable decrease in production.
I also saw that the previous poster mentioned fenugreek. It really does work - you can get it at GNC or a healthfood store and it usually comes in 580-610 mg caplets. You take 2 at breakfast, lunch, and dinner. A friend of mine who pumps exclusively said that it increased her milk supply by 2-3 oz per side per pumping session, and that happened within 3 days! I recently used it when I needed to pump and freeze a stash for when I went on my first trip away from baby, and it did seem to increase how much I was able to pump immediately after feeding baby.
The other thing is to make sure you are getting adequate stimulation in the morning and at night to keep your supply up. Maybe do an extra feeding or 2 when you are with your baby to help boost production, and/or pump immediately after feeding your baby until you empty your breasts. When your breasts are completely emptied it tells your body to make more milk. One way to help with this is to pump one side at a time and use the opposite hand to massage the breast and make sure all the ducts are being emptied - takes a little longer but it is well worth the results!
Best wishes! I hope you continue to have the joy of breastfeeding your son! Let us know what you find out! :o)