I have the same amount for my youngest as my eldest it’s £500 for both. You get £500 per quarter so really it’s only £166 per month. Let’s saying you use a top up worth £200 in month 1 then £200 in month 2 you will only have £100 left for month 3. Then in month 4 it will reset.
It's up to £500 per quarter as a maximum and it has to be up to 20% of your bill.
Just a guess but are you part way through your three month confirmation period so it's less until it resets again? I'd be interested to know why this would be anyway as due baby no. 2 soon and was hoping to utilise the same entitlement.