Some people have nipples that stretch while pumping so that they extend to the end (or almost to the end) of the flange tunnel. When this happens, too much areola is pulled into the tunnel, which compresses the milk ducts and reduces milk flow.
What can you do if you have elastic nipples?
Silicone Pumpin Pals are a breast shield that are shaped differently than most other flanges. Instead of the ring at the bottom of the funnel, the cone tapers into the tunnel. The softer silicone can also be helpful for elastic tissue.
Beaugen Breast Pump Cushions are inserts that go inside breast shields. They are intended to provide a more comfortable pumping experience, and can also help if your optimal flange size falls between two sizes (It reduces the flange size by about 2 mm). Many people have found that Beaugen cushions helped with elastic nipples, both as a result of the smaller size and the material. Maymom makes a similar product with many sizes which fit into most pumps including wearable models (if you get the shorter version).
LacTeck BabyMotion Flanges are made of a softer silicone and tend to pull less areola down the tube.
Lubrication can be helpful for some pumpers to prevent nipple stretching and makes pumping more efficient. Use a water-based lubricant or non-sticky nipple salve.