Alright, so I finished all APR missions, and then finished 2 of the UFLL missions. I decided to find all diamonds next, along with all the lost buddies and safe houses. Now, I finished all the buddy missions only to find that the UFLL door is locked. I saved downstairs in the UFLL HQ and exited the game. I am playing on ps3, and have NO game saves prior to the door closing. Does anyone have a fix? because I really dont want to find 112 diamonds again.

Have you finished every other possible mission? (Buddy, Telephone-post, etc.) I think i had the same problem (it's been a while), but after i completed another mission first, the door was open.
Hope that helps, good luck!

I have finished every possible mission, buddy, tower, Found all buddies, found all diamonds but not all safe houses or all outposts.

Doesn't sound good. I don't think open a safehouse will help. Try to save the game outside of town, than walk into HQ and try again, before you give up, but... .

I had this problem. To solve it I went to mikes bar and received a mission from the girl buddy to assassinate 2 men trafficking hazelenut drugs into the area... after completing this mission the door upstairs was open and I received the mission to assassinate the leader of the other faction.. hope this helps