SAP Note 22977 - Contracts are not the source of supply list

Symptom:

If you create your own document type for purchase requisitions with transaction OMEB (with or without reference), existing contracts are not proposed during the source determination (ME56).

Cause and prerequisites

'X' is missing in table T161A-LPET in field LPET (release orders and schedulings are allowed) for the new document type and the reference document types. However, this cannot be maintained via OMEB.

Solution
Call SM31 for table T161A. Enter 'X' in field ROrd/sched whereever required.