Consumer Reports has a couple of pages of information about ventless units. They are bad and dangerous, and many building codes outlaw them. They pollute the indoor air. They consume oxygen. They are a safety hazard. I suspect your experience is not unusual with these units.

I am no techno or expert, but what you described is exactly what our fireplace retailer just described would happen to my ventless fireplace logs because they are 7 years old. He said no brand is designed to last that long and you can tell if they are old and need replaced if they start to have a bad smell and they don't stay lit. Mine is not having that issue at all so I just posted asking about the lifespan of ventless. We'll see what the experts say on the longevity of ventless units. We have loved our over the past 7 years, stays lit, heats the whole house, and never a smell. We keep the carbon monixide unit about 3 feet from the fireplace and it has never alarmed. Do you know what age your logs are?

Best wishes,
MemphisREI