I doubt that the glow plug heat range is the problem. The heat range of the plug effects the engine timing a bit but shouldn't have any effect on starting. It could be that the glow plug has gone bad. Try a new one of the same type. It may be the battery in the igniter which can't put out much current at such low temperatures. Be sure it's fully charged and keep it in your pocket to keep it warm until you attach it for starting. If you have a power panel, use the glow plug connection with an appropriate plug connector instead of an igniter. If you still have starting problems, you might try priming with a little ether at the carburetor. This can be done using a squirt bottle (starting fluid with ether is available at wal-mart and auto parts stores).