If the pressure drops to allow one to exhale easier, some brands kick the high pressure back in faster than others, resulting in feeling like the breaths are short and shallow. There are machines with a-flex,
meaning it auto adjusts to your exhalations needs. A bilevel allows setting the pressure and timing of inhale and exhale and the time in between.
Keep communicating closely with your doctor and work thru this till it is working well for you.