A possibility is that your OMAD is leaving you with less than optimal glycogen stores, yet not close enough to depletion levels for the liver to kick in and start ripping apart fat to get at those ketones. Years ago we referred to this as “No Man’s Land” and it is a pretty sucky, gray landscape to camp in. Recently I was rereading “The Hobbit” and realized the journey through Mirkwood is a good analogy for No Man’s Land.
So my advice for you is to GET THE HELL OUTTA MIRKWOOD! Ha! You can go forward or backward but may have pitched your tent and set up camp in a disadvantageous spot, my friend. Go forward by extending your fast into 36+ hours or start adding in a bunch of cardio to blow through your glycogen and win some nice clean Ketosis. Or go back and start pounding whole food plant starches during your eating window, like sweet potatoes, oats, barley, brown rice, whole grain bread, etc. Give either strategy a try and see what happens because a good OMAD regimen should have you feeling like “a god amongst men”. Cheers and good luck!
Are you drinking plenty of electrolytes during the longer fasting periods? I found after starting vitamin D supplements and drinking white tea my extended fasts were so much easier. I also found drinking a pint of water would curb any unexpected days when I’d dwell on food constantly. When I first started the longer fasts I’d treat myself with a thin slice of jalapeño just for the taste distraction when I couldn’t shake the food thoughts too.