If you want to pump more, there are two areas to reevaluate: your pump (and how you use it) and your body's overall milk supply. When it comes to the pump, it is important to have a good quality double pump that is meant for everyday use. These pumps are designed to maintain your milk supply even when you're separated from your baby everyday. It's important that the pump is in good working condition and that the parts you are using are properly fitted and well maintained. Contact your pump manufacturer if you are unsure whether your pump is functioning well.
Some pumps allow you to adjust both the suction level and the speed with which the pump sucks/releases; other pumps just allow you to select the suction. You should always keep the suction at a comfortable level for you. More suction does NOT equal more milk and can actually cause pain and damage if it's set too high.
If your pump allows you to adjust its speed (cycles), then it often helps to start with a fast speed to mimic those first few sucks that your baby does to get the milk flowing. Once your milk begins to flow, then turn the speed down a bit to mimic the long, slow sucks your baby does to drain your breast. You can adjust this speed from fast to slow again several times during your pumping session to help get the most milk possible. Some pumps will automatically make these changes in speed though, so you might not even need to bother.
As for your overall milk supply, sometimes going back to work changes your schedule in such a way that you're no longer draining your breasts as many times throughout the day as you were before you went back to work. This can cause your overall milk supply to start to drop, and in turn, many moms notice that they're able to pump lesser amounts of milk while at work. If this starts to happen, take a step back and take a look at the big picture. Are you still draining your breasts (either by nursing or with the pump) at least seven to eight times per 24 hours? If the total number of feedings and pumping sessions has dropped to six or fewer times in 24 hours, your body may have begun to assume that your baby is beginning to wean and your milk supply is dipping because of that. Adding in a few extra nursing or pumping sessions will help with this, but it will take a few days for your body to increase production again, so be patient.
Other questions to ask: Have you started on any medications or birth control lately? Has your period returned? Has your baby changed his nursing patterns or started sleeping longer at night? Any chance you could be pregnant? All of these can affect your overall supply. If you think any of these might be a factor, consulting with your local lactation consultant (IBCLC) and your own doctor may help you find the answers that work best for you.