Purchase charge type can be added to sales order, but cannot be billed from the sales order

All three charges are showing on the sales order (one is a non inventory item brought in by Purchase charge type). The two other lines can be billed by clicking Next Bill or Bill Remaining, but the non inventory item can't be billed that way, even after that item is fulfilled. If I skip the sales order process, and just create invoice for the project and bring in all charge items, then the non inventory item / purchase charge can be invoiced just fine. It's just somehow I cannot bill it from the sales order, like the other time based