Hi - my 11 yo daughter has sleep apnea.  My oldest (53) brother has sleep apnea and has had surgery and uses a cpap for the last 3 years.  He is a major sleep apnea advocate.  He also has problems with allergies.  He suggests that the nasal passages are inflamed and swollen from allergies and you don't get go air flow.  After listening to his history I'm pretty sure that my daughter also has allergies.  I plan on getting her tested.  
Any thoughts on this subjest appreciated!

Allergies and asthma should be well controlled for cpap to do it's best.  Many apnea patients use steriodal nasal sprays, such as rhinocort, flonase, etc. on a regular basis, in addition to the cpap. A heated humidifier is also very important in minimizing nasal tissue dryness and inflammation.