Hello,
here is a wiki page regarding Shopping Cart transfer in Classic Scenario:
Transfer process of the Shopping Cart
Check also its child pages.
Maybe, you will have to debug to understand what is the problem.
Use those KBA:
1943620 - How to create test data in ECC system (from SRM)
2134946 - A Sample of BAPI_PO_CREATE1 Test Data for Material PO Transferred from SRM
Regards.
Laurent.