Hello
Check the following note, which explains the most common causes for this kind of issue:
http://service.sap.com/sap/support/notes/1967840
The document below may also be useful:
MRP: Vendor is not selected and general problems on source determination
If you are using an MRP group, check the creation indicator on transaction OPPR and make sure that it is set to generate schedule lines.
BR
Caetano
