Chances are the pressure is too high, for what will travel through the coupling. When the pressure is too high, it will lock the ball on the return and one of the hoses will sometimes pop out or it just stays locked until the pressure is released. That could mean faulty couplers or the setting on the tractor is too high.

 One of our suppliers changed brands of couplers once, and probably close to half of the couplers had to be changed on several pieces of equipment. After that experience, we always make sure that we have good couplers.