Make sure that the user you are testing this with, has all the necessary roles and authorizations on the portal to Start a BPM Process.
For all possible roles and authorizations related to SAP BPM, refer the following:
http://help.sap.com/saphelp_nw73/helpdata/en/45/d7d0e08a164c5e87e4604ba89c632a/content.htm
https://help.sap.com/saphelp_nw73/helpdata/en/9c/23eaeb4c53486f8d9c4cb376b99994/content.htm