Yes the motor may not be functioning efficiently, or maybe you need some new parts. Or sometimes if you change worn out membranes (the white membranes) you get better results. (you can get them at medela.com or mybreastpump.com)
The best time to pump is first thing in the morning, because during the night you have a breastmilk proudction serge. Or nurse on one breast while pumping the other.
Maybe try hand expression or buy a Avent manual pump - which is an inexpensive yet effective manual pump-to see if you get better results than your PIS. Different things work for different women.
This is from the LLI website:
The Marmet Technique of Manual Expression*
This technique has been developed by Chele Marmet, a La Leche League Leader and lactation consultant who is the Director of the Lactation Institute in West Los Angeles, USA. As with any manual skill, practice is important.
1. Position the thumb and first two fingers about l" to 1 1/2" behind the nipple. Use this measurement, which is not necessarily the outer edge of the areola, as a guide. The areola varies in size from one woman to another. Place the thumb pad above the nipple and the finger pads below to form a "C." Avoid cupping the breast.
2. Push straight into the chest wall. Avoid spreading the fingers apart. For large breasts, first lift and then push into the chest wall.
3. Roll thumb and fingers forward as if making thumb and fingerprints at the same time. The rolling motion of the thumb and fingers compresses and empties the milk reservoirs without hurting sensitive breast tissue.
4. Repeat rhythmically to drain the reservoirs. Position, push, roll; position, push, roll.
5. Rotate the thumb and finger position to milk the other reservoirs. Use both hands on each breast.