QB Issue Resolution:

If I understand it correctly, you’re trying to add your billable cost to your invoices. If my assumption is right and those entries are not available when you click on “Add Time/Costs”, then, it would be best to run the Unbilled Cost by Job report to check if it is still listed there.

If not, then, there is a possibility that the billable cost is probably billed already to the customer invoices, not marked as billed or deleted from QB.

You may pull up this report by going to Reports menu> Jobs, Time & Mileage> Unbilled Costs by Job.

Resolution for Issue 'My billing is missing entries when its time to make an invoice. How fix problem, and how far do I need to go back in my books to find the missing reimbursement?' available: Yes (Solved).
Source: Intuit Community forum.

My billing is missing entries when its time to make an invoice. How fix problem, and how far do I need to go back in my books to find the missing reimbursement?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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