Why don't i ever break bosses stances?

Im not really sure what im doing wrong. Im not talking about parrying or anything like that, im talking about when you hit a boss repeatedly their stance breaks and u can do a critical hit. Im using a str/faith build and according to many build guides im supposed to be stance breaking bosses like crazy with the sacred bloodfiends arm. But ive fought every boss in the DLC woth this build and the only boss' stance that i have broken is rominas once during the fight. Im using a charged heavy attack build with the stance breaking cracked tear(forgot its name) and the AoW RKR. Im on ng+13, but even then their stance should be breaking, no? Is there something im missing?