I have successfully completed "There Stands The Grass" involving Dr. Hildern with no problems, but now I have at least three quests and a potential follower that are locked because the fine Doctor will not engage propper dialog with me!

The following quests are effectively locked for me:

-Birds Of A Feather

-You Can Depend On Me

-Heartache By The Number

-Follower "Cass"

My active quest note tells me to: "deliver Crimson Caravan invoice to Doctor Hildern at Camp McCarran" -but when I attempt to do this by initiating conversation with the Doc, he says: "Hello, what can I do for you" -but my only option for response is "Good buy" or "I have to get going" or the like! I've tried everything I can think of, but none of the quests I mentioned seem to be able to progress unless I can get this guy to accept my invoice!

* I am in good standing with the NCR, and have not attacked, or done anything that I can think of to offend this carractor, I can't figure out what the problem could be!? *

Any ideas on how to get around this???