2000 Ford F-150 Truck won't start

Hil, I have a 2000 F-150 4.6L with about 35,000 miles. I keep the truck in the garage and during the winter months drive it around once a week for about 1/2 and hour. This past week the temperature in the garage is about 20 degrees. I tried to start it and it was a no go. The starter cranks the engine fine. Last year with a similair cold spell, it did exactly the same thing. With the battery being 6 years old, I replaced it and all was fine until today. I have let it sit previously for a week or 2 during the winter and it started right up, so i'm a bit confused why this would happen once in awhile? Any ideas would be appreciated, thanks.