Your LP Regulator is NOT providing correct system pressure which is 11" WC
Furnace is biggest propane user and when system pressure is low it will not light.
Have the LP System pressure checked and adjusted to 11" WC with at least 50% of propane appliances in service.
And there is no AIR in system. All air was purged the first time propane tank was filled.
During periods of non use the regulator has no demand and remains closed.
When first using system the flow needs to be established and a demand placed on regulator.
Sometimes, because pressure is low, it will take several attempts at lighting off to establish that flow and lace the demand on regulator so it is in service.