Cough ist machine?

Hi, yesterday i got a cough ist machine given to me by the hospital because i have spinal muscular atrophy and have a weak cough which means if i get a chest infection i cant clear the secretions easily. The physio shown me how to use it and sorted out my settings. I have the pressures on 30 inhale and 30 exhale. Now i am home and have had a few practices on it, i have made sure that all of the settings are right, when i test it by covering the end of the tube with my thumb the pressures are right (it goes to inhale 30 and then exhale 30) but when i put it over my face and do it with the mask, it only goes to just past 20. Am i doing something wrong? Is it because my lungs cant take anymore air in? Or could it be how my mum is holding the mask? In the hospital when the physio did it on me it went all the way to 30 so i'm a bit confused. Thanks for your help