Careful with your terminology. This is not a repeating field, but a field shown in a portal to the lineItems table. Have you made any changes to the relationship between purchase orders and line items in manage | Database | Relationships?
If "allow creation of records via this relationship" is no longer specified for the LineItems table occurrence in this relationship, you will not be able to add new records in the portal as you could previously. (That's what happens when you add data for "2nd, 3rd and more items".)