If for any reason you're having trouble pumping enough milk, the first thing to do is check your pump.

Replacing Valves:
You pump creates suction with the use of valves - most of which are attached to the flanges where the milk flows from the flange into the bottle. These valves are NOT intended to last the life of your pump - they are designed to be replaced every six months. If you have been using your pump for more than four months, or if you bought it used, the first thing to try is replacing the valves. You can get new valves where you bought your pump, or at many online sources (here's one, here's another - I just did a search for "medela membrane" on google and came up with dozens, several with free shipping).
What are the valves?
PIS: The small white flippy membranes on a PIS attach to a yellow plastic piece - these two together make up the valve, although the yellow plastic piece is actually called the valve, the white flippy piece is called the membrane. .
PY: The white cone-shaped pieces that are about 1/2 inch long and made of soft rubber - these are the valves.
Isis: The valve is the little white rubber piece about the size of a dime that has the six-pointed star on one side. If you're replacing the valve on an Isis, you should also replace the soft clear piece that fits above it. If your Isis is not working, this valve may be in upside down. The star should point up (remember, stars are in the sky - the sky is up).
All other pumps will have a similar system.

Check for Cracks
My Isis stopped working when it got enough cracks around where the bottle attaches to reduce the suction. With an Isis, there's not much to do but buy a new pump, but with an electric pump, you can replace just the flanges for minimal cost. Hold them up to a bright light and examine them carefully for small hairline cracks. If you consistently are cracking your flanges, don't screw them on to the bottles so tightly. The suction in a PIS and PY is created above the point where the bottle attaches - the bottle is not part of the vacuum, so does not have to be attached tightly at all.

Have your pump tested
If you've replaced the valves and found no cracks, it's a good idea to get your pump tested. This is the advantage of buying your pump locally from a Lactation Consultant or specialty pump rental station - they will have the equipment to test your pump's suction to see if it's still working correctly. Even if you didn't buy your pump locally, you should be able to get it checked. Look in the phone book under "Lactation", or call the hospital maternity floor to find out who rents and sells pumps in your area.
Most pumps are designed for one year of full-time pumping. If you've done more than that, the motor may be conking out, and there's really nothing you can do. One day an enterprising person will start replacing the motors for mothers who pump for more than one baby, but until then, they go into the landfill and you need to buy another one. If you want to feel better about this, go and price those big cans of DHA-added formula - you're still saving money, even if you buy a new pump for each baby.