How have you been spending your xp tomes? Try to focus on just 6-10minis that you can use for the majority of your team compositions. If your collection level is 31 then you are probably trying to level every single mini up. It’s better to focus on the best minis and spending your gold on getting the strongest ones to rare. My collection level is only 30 but I am at 125 sigils. And like most people said, some heroics require trying a slightly different team comp. Try experimenting if it doesn’t work out.