PDF attachment to wf item not always found

My ABAP program creates a Smartform report which I convert to PDF and then use fm SO_NEW_DOCUMENT_ATT_SEND_API1 to send to the user's document folder. I use the NEW_OBJECT_ID parameter to build the object key for fm SAP_WAPI_CREATE_EVENT. Using debug, the correct object id is being passed to the fm and it matches the entry in table SOFM. The wf is started and the pdf is attached to the inbox item. But when I click on the attachment to open it, sometimes it is found, sometimes it is not found, and sometimes it opens the attachment from another wf item. Sometimes, if I close Business Workplace then start it again, the attachment will then be found - but this is not consistent either. Any suggestions? Thanks.