I struggled with this when I first started - the good old STALL ... that is when you have your PIT at temp and the meat seams to take FOREVER to get past a temp to the one you want. I basically learned after 2 pork shoulders, 1 brisket, and a tri-tip - It's Done When It's Done.
Each piece of meat is different, more or less fat, leaner, tighter, however you want to describe it. And usually they take about the same amount of time but there are those pieces of meat that are just, well stubborn! Just got to be patient and wait it out.