What you may be experiencing:
QuickBooks Desktop Point of Sale V18 customers, have noticed some latency when processing sales for their customers that involve numerous transactions (e.g. counter customer, cash customer, etc.).
What you can try now:
You can create a new customer with a different name to help alleviate any indexing issues that could be causing this behavior. You may still use the original customer record for checking historical records.
We are continuing to research root cause. Currently, we do not have any additional information to provide. This issue will remain open, but there is no set plan to provide further updates. Once we have new and relevant information, we will communicate that to you.