Creating deliverable milestones within a work activity....explained below.

I have an activity of work which has a defined begining date and duration. For this work there are several deliverable document items. I have set these as start miletones and related to the work activity with a start to start relationshbip.  The documents themselves don't alweays have successors but I don't want to leave an open end and thought i would coule make a Finish to Finish activity with the work activity. However P6 not letting me do that. I have never come accross this and not sure how to deal with it. i do need to show the deliverables as activities with milestone dates to track progress and of course some do have significant successor ties but not all.   

any guidance / suggestions appreciated.

work activity (5 weeks) 

SS deliverable milestone A

SS deliverable milestone B

SS Deliverable milestone C

a,b, and c need to be completed before the work activity can end.