Seems like you just might be getting unlucky. My HT has similar potency and its breach usually sticks, and only occasionally resisted (except in Geo TB, where it almost always is). There's a base 15% chance for an enemy ship to resist debuffs, and certain ships can boost tenacity or reduce potency as well (Rex ARC boosts GR tencity by 10%, ghost decreases all enemies' potency by 25%, chimera boosts tenacity by 10% for all allies each time an enemy is defeated).
I'm guessing all ships involved are L85? Difference in ship level also affects potency if your ship is lower than the enemy ship's level.