Custom Line Item (CLI) is not applying a charge to a Billing Account in a certain month within customer tenant.
One possible reason could be the absence of billing artifacts for the billing account in question during the target month. You can verify this by filtering Cost History report by the Billing Account in question and check if there are no costs for the target month.
The CLI charge amount is added only if the account/subscription/project has some cost in the particular month. Having some cost has been an indicator for the account being active and hence we do it this way.
For WS3 customers for AWS and Azure we had rolled out a feature which lets us add a flat amount to account/subscription even if they have zero cost, but its not there for GCP yet.
Apply the Custom Line Item to a month where the Billing Account have incurred costs.