Why can't I pump?

Perfect Synch. If you mean you pump properly with the right equipment and you obtain little milk, then this is the most likely answer. If your baby is healthy and following good growth parameters, it probably means you and the baby are in synch. That is , you are producing just what the baby needs. No more and no less. Thus you are drained after each feeding with nothing left to pump. A perfect partnership!
Right Environment? Many mother choose to pump breast milk for various reasons. When you do so you HAV eto ensure that the environment is right. If you are at work, find a very quiet room where you can relax. Look at pictures or videos of your infant to stimulate the release of milk. Be sure that the pump is appropriately fitted. You should also ensure that you are taking in lots of fluid throughout the day.
Baby Trumps the Pump. A baby is always more effecient than a pump at getting milk!