It is the long-standing tradition in the conventions of an organization of which I am a member to not allow someone who speaking to a main motion a make a motion at the end of a speech during debate (such as a motion to amend or previous question).
Our president asked me if I could find the rule on that, and I couldn't. My guess was that when you are recognized you can debate or make a motion but not both. I couldn't find anything that gave a clear answer.
So, is our rule something that's in Robert's Rules or is it just a long standing custom of our organization?