I installed a Heatilator Caliber, natural gas fireplace, model CNXT4236 a couple of years ago in my basement. When the outside temperature gets below 32°F or so I have to heat the area where all of the electrical and piping is below the firebox to get it to light. The pilot comes on and the sensor glows red but the burner won't come on. If I blow an electric heater in the area for a few minutes it lights fine and continues to work fine until it's been off for several hours.
From the Manual, one of these two solutions sounds most likely:
The fact that it occurs only when cold could mean a poor electrical connection (metal expands when heated which results in a better connection), or the thermocouple is not properly in the flame (maybe by such a small distance that the room heat allows it to get to the proper voltage).