If you are unable to top up your meter for a long period of time, your meter will build up standing charge debt. Standing charge is the cost of keeping you connected to the network. The fee is taken from your meter every day, even if you're not using gas or electricity.
So if you top up your meter after a long period of time, and your credit doesn't last as long as you were expecting, some of it may have been taken to repay standing charge debt.
You can look up the cost of your daily standing charge in your Welcome Pack.