I experienced the same problem during our 5 week summer vacation. Turned on the heat at the thermostat and the furnace fan came on sounding like it would light but just kept the fan running. My first thought was that it might be something to do with my digital thermostat as I recently installed one this year. As time progressed during the holiday I noticed that when switched to the other tank everything ran fine. When on the opposite tank, everything but the furnace ran. I talked to a buddy of mine and he figures that it is something to do with the propane tank itself and to burn of all of the gas in that tank and since he is certified to work on these things he will change the valve.
I suspect that maybe the valve is not allowing enough gas or too much and the furnace may have some sort of sensing unit that recognizes this and will not ignite the gas. (fan just keeps running and will not ignite). I will let you know what happens when we change that bottles valve.