It is freezing up on you. Either put your tank in a bath of water or just change it out. Propane is generally sold by volume, not per tank. Hope this helps.
In NZ LPG (propane) is actually a mix of propane and butane and its usually the butane that causes problems in the cold. I think they adjust the mix throughout the year. I think the butane is cheaper hence the mix. This means a bottle filled in summer might not be so good in winter. Could me the same elsewhere i guess
Propane (and butane) is a refrigerant gas. As it changes from liquid to gas it cools the liquid and then the pressure drops. Set the propane cylinder in a tub of warm water to offset the cooling and your pressure stays up.