What pieces are you missing? Do the bosses you are killing drop those pieces? Bosses can only drop pieces in their loot pool. If you have exhausted that boss' loot pool, you will get repeats, even if there are pieces you still need that only other bosses drop.
10 I have Bahsei, 7 I have Sax, 10 I have Stone, 8 I have Sul, just as you look down through the perfected sets.
What pieces from those sets? For example, weapons and jewelry are only in loot pool of final boss. So if all you need are weapons and jewelry, every piece of gear dropped in trial will be a repeat until you get to final boss.
And the other bosses have specific armor pools too. Basks-in-Snakes can only give four specific armor pieces from each set, for example, not all seven. You can’t just farm him for all the body pieces even, much less the jewelry and weapons.