znetwebhost
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Did you checked their service status ? verify that it is active and a proper due date is set.
Some customer sign up for automatic billing therefore they would't usually notice if they do actually pay or not especially if the amount is little.
Can't really blame the customer at this point, but you can certainly have them pay back for the miss months.
I have to agree with Conor and Blue and many others here.
At the end of the day, it's your responsibility to make sure invoices are done correctly, and if WHMCS is not generating them properly, it's configuration needs to be looked at.