Yesterday when I was playing I was just getting ready to start Making a Mark. The only safehouse that I had unlocked was the Beach House, since I wanted to leave em' all for after this mission. I passed it and decided to go to the safehouse in Old Bullworth Vale. When Jimmy got up for some reason the only challenge that I could do was the Greasers one. After passing Finding Johnny Vincent I went to 'take over' the Townies' one. The Nerds' and Jock's challenges remain unavailable.
