Many people have comfort and size issues when it comes to using a CPAP machine. Since sleeping with a mask on your face can feel very intrusive and bothersome, it takes time to find a mask that fits well and offers enough comfort to allow the patient to fall asleep.
Masks often get in the way or don't seal correctly against your face, which causes the CPAP machine not to do its job correctly. Some people even describe a feeling of claustrophobia when wearing a CPAP mask. Others have a hard time finding one that fits properly, and isn't too loose or too constricting.