S9 hard to breathe out?

One of my S9 units is set as my others but it is hard to breathe out. Any sugestions, is this comon?

Verify that it actually is set identically to the others, including EPR.

You can compare it's pressure to your other devices by building a simple manometer. Essentially, putting the end of the tube into a deep water bath, turning it on and then measureing how far the tube goes down before the air stop coming out. if the devices are all set the same, they should all measure the same.