Data Validation: Error alert not displaying when invalid data is entered.

I created a workbook for training purposes; in one worksheet containing an Order Records in an Excel Table. The column labeled Inv_No has data validation applied limiting the invoice numbers from the invoice number worksheet. Whenever an invoice number is entered into the Order Record table the list of available records removes the used invoice number. Therefore making it unavailable to the in cell drop down list in the Inv_No column. The problem is that initially the error alert would pop up when invalid data was entered and I could choose whether or not to allow a duplicate invoice number. But now I

neither receive and Error Alert nor does the data validation disallow invalid data. At one time I copied the table to a new sheet and deleted the old sheet and renamed the new sheet the same as the old one and it worked for a while but now it back to not displaying Error Alerts or disallowing invalid data.

One other note if this makes any difference, I am sharing this workbook with one other person who has never edited this particular worksheet. Also there are other columns in this worksheet and other areas in this workbook that have Data Validation and all of those are working as advertised.

The Data Validation criteria is set as the following:

Settings:

Allow: List

Source: AvailinNo (which is the range in the Inv Numbers Sheet.

Input Message:

Show input message when cell is selected. is checked.

There is a Title and an input message and that's working as expected.

Error Alert:

Show Error alert after invalid date is entered. is checked

Style: set to Warning

I have a custom Title and a custom error message.