Having a romance initiated with one follower (whether you're aware of having done so or not) sometimes precludes initiating the same with another follower, and the game is not clear in informing you when flirting has successfully transitioned to active romance. For example, I was just being friendly with Dorian on my second playthough (male wizard on nightmare mode), and in short order I was suddenly cornered by a vaguely disapproving Mother Giselle, and it took me a few moments to realize I'd inadvertantly triggered his romance questline (confronting his father in redcliffe), I did the quest, and then broke it off to romance cassandra instead.
So, my advice ... two things. First is that new conversation options and the ability to advance side quests with followers usually occurs each time you complete a quest under the main story mission ("Inquisitor's path"), so always make the rounds of all your follows when doing so, and talk to each of them at least twice while doing so. Second is that the single most powerful way to rack up approval with multiple followers at once is to do the 'sit in judgement' quests, and (in most but not all instances) choose the option to do pennance in service to the inquisition or the people they wronged.
Last, for bull, is to also be sure to talk though all possible options with his chargers aide, who usually sits or stands nearby.
BTW, amusing aside ... if you get a romance going with a follower other than bull and dorian, bull and dorian with sometimes begin a relationship with each other if you have them in your regular party out in the field. Some of their dialog is amusing.
