This is likely to be because the load increase is too high and increased too quickly. Work on four weeks before rushing on to the higher levels. If this is the first time you have done any form of Inspiratory Muscle training, it is good to note that the device has a variable resistance, think about ¼ turns, rather than an increase of a whole increments.
We generally look at the POWERbreathe mechanical units as lifting weights or dumbbells.
Imagine lifting 35-Kilo’s and being unable to lift 40kg. If you then try 45kg, 50kg the effect is the same as the 40kg, ultimately the load becomes irrelevant.
Focus on Quality - Breathe in with rapid Airflow, continue until your lungs are full, then breath out slowly, good technique will be better than focusing on the resistance setting to which you are working. Check the POWERbreathe manual, or read on line at www.powerbreathe.com
Do not train to fail,
Utilise your POWERbreathe effectively, and get the best results, you should be able to take 30 complete breaths in your training session. A full breathe thirty times under a reduced load is better than short breaths and straining under a heavy load, (20Kg dumbbell through a full Arm Curl x 30 reps, rather than the half curl with 35Kg and only completing 20 reps.