QB Issue Resolution:

Perhaps they are marked as inactive and you don’t have inactive customers showing? Make sure the view menu at the bottom of the customer center has All Customers showing. Then you may want to sort it by name or filter it looking for the missing customer.

If that doesn’t work, try File->Utilities->Rebuild or Edit->Resort list while the customer center is showing.

Resolution for Issue 'I have customers that do not show up on my customer list. When I try to add them, I get an error that I cannot since they already exist. What should I do?' available: Yes (Solved).
Source: Intuit Community forum.

I have customers that do not show up on my customer list. When I try to add them, I get an error that I cannot since they already exist. What should I do?: 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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