I don't know how well this formula works but I had the same issue until I tried a couple of things. First Luck items, some people say they are useless but to me it has made a difference its up to you to try it. Second if your DPS is low sometimes it seems to affect RNG. As a tank it's usually pretty high and as healer. But as a range or close DPS its seems to effect it somewhat if people are contributing more. Just a tip.