A few things to check to determine what may be going on;
1. Spring tension - when you lock the levers down, is there plenty of tension from the springs or do the "flop" over
2. Pin wear - are the ends of the pins worn allowing the attachment to slip off
3. Pin engagement - When the implement is locked on, what portion of the angled section of the pin is contacting the edge of the "hole" in the implement? Mine are about the middle of the angles section.
If pins are sitting too shallow into the "holes", may need to open up the hole a little to get the proper engagement.
If pins are bottoming out and not holding implement tightly, you may need to weld a plate beside the implement "hole" to tighten it up.