Why? Two or three clusters of 4 to 5 OA/HA each night. Even with cervical collar?

I regularly have these clusters which represent the bulk of my total AHI. Putting on a cervical collar does not change anything.
What should I check/change ?

Please let me know your opinion.

THANKS

As mentioned in a previous thread, other possible fixes include using a low, firm pillow and staying off your back. But perhaps those are not relevant for you?

It'd be helpful to see a chart; could you post a recent one? You'll need to use an image-hosting site like Imgur.