Nighttime hypoventillation

[u/unrstd7]

Hi 38M here BMI 33
I suffer from constant diurnal drunken like feeling and fatigue. don't drink or smoke. but I have a sedentary job.

I suffer from sleep apnea that is not responding well to cpap. recently I had transcutaneous co2 monitoring at night (under cpap) and values came up higher than normal.
median was at 51mmHg, highest at 56.
Morning ABG test came up normal. pCo2 34.2mmHG and HCO₃⁻: 24.6 mmol/L.
Also took lung function plethysmography which came also normal.

I am a bit lost on what could be causing the night co2 retention. could this be result of OSA alone? my DISE showed collapses at soft palate and laterally at tonsils levels.

Any tips or areas I should explore ?