Cannot resolve charge account on internal requisition, but works ok on Std PO using same item

I am using the same inventory item on three document types.

Standard Purchase Order. Item works correctly, account generator resolves the charge account, all is well.

Internal and Standard Requisitions. Same destination as PO above, same item, same preparer, but cannot resolve charge account on the distribution - APP-PO-14082 error Charge Account is Mandatory and could not be determined

Cannot understand what would cause a difference. Any suggestions most appreciated.

Many thanks