My potterton prima f60 will not relight itself?

If the gas runs out and i have to put more money on the meter when i come to restarting the boiler it just keeps powering up then shuts down, I can hear the pump pushing the water through the pipes but it sometimes takes hours to relight. The button below the boiler stays up, I just have to keep turning the dial from o to 1 and at some point over 24 hours relights itself. Im typing this question cos the unit has no problem when it has powered up and goes for months unless i let the gas run out then i have to go through this proceedure again.