40f to 50f is great weather for starting.

my friends and i run in 20f to 30f all the time. just hit the motor with a hairdryer or heatgun for a couple of mins then take it outside and start it up before motor cools off (don't get hasty and forget to turn on your radio).

we haven't had any problems starting yet. well except for bad glowplugs.

maybe you should check the glow plug. might be starting to go bad