Make sure that you're putting the mask on appropriately. If you have a bed partner, are they complaining of snoring or any mask leaks? Its possible the mask isn't fitting appropriately on the days your AHI is high.
has anything changed to cause a rise in ahi stress can cause all types of malidies
things to consider are births marragess and death divorce etc you get the idea
also
other health problems may impact on your ahi and you should look for medical advice in this area
are you maintaining your machine like changing the filter
It's most likely that the prescribed pressure was determined on a night that you weren't at your worst from an OSA standpoint. Now, on those nights, you don't have enough pressure. Talk to your sleep doc and see if he/she will bump up your pressure a bit or let you change to a CPAP auto.
What is your flex or EPR set to? By turning off this technology you will raise the average pressure that you receive. If your numbers go down then this might be evident of a need for a higher pressure. If they stay the same or get worse then your problem is probably a leak of some sort. It is alos important to document weather patterns on the nights that you have higher numbers. Outside humidity and barometric pressure changes can greatly affect your treatment.