Therapy will be far less effective if you have nasal congestion. Nasal congestion can be caused by several things. Seasonal colds and flu, hay fever and even your CPAP therapy can cause congestion. Congestion from CPAP is caused by the flow of dry, pressurized air through the sinuses, and can be particularly common for those with frequent sinus infections.
Congestion can be relieved with a nasal saline spray to moisturize your nasal passages or frequent use of a nasal irrigation. Regular sinus irrigation will keep your nasal passages and sinuses clear, moisturised, and healthy. Not only does it flush out bacteria and viruses if you have a nasal or sinus infection, but it also removes them before they have a chance to enter your system.
For long term care you should consider adding a CPAP humidifier to your CPAP machine.
If you suffer from long term sinus issues you should consider changing your mask to a full face mask.