This will be on forum and has been said many times before , once you do a solo story quest they will lock un till all completed . To open more tiers of solo player story , you have to do Government quests in multiplayer up to at least mines. Once these requirements are met you will then be able to re do any story mission after in solo play .
Most of us would only complete a few in order to open up dr osto mission and will not complete mission so it does not lock , this way we can convert our enemy parts without finishing it all