You are trying to obtain a specific item from two retainers who live solely off RNG. This is like trying to shoot at the center of a target 100 yards away with a shotgun, and wondering why you aren't even hitting the target.

This is normal for the retainer system, and expecting this to be a return each time would be delusional. Just make sure they are well equipped retainers; their gear rating does affect chance%. It will come to you!