I know this may sound crazy but check the simple stuff first. Load tools in and out by hand watching the clamp and unclamp signals in the diagnostics. Also during tool changes monitor the ORAR signal in the spindle monitor screen to see that your spindle is maintaining the m19 state. I have seen this happen on machines if the spindle is twitching slightly causing the orar(orientation arrival) signal to flicker. However i do not have one of these machines we have the mazak integrex version but tool changers are often alot alike. The other thing i have had on Kiwa machines is wear in the gear box causing this. Most of the tool change gearboxes are made with a pair of roller balls that ride in grooves as the motor rotates and is actually what turns the double arm and also pulls the tool out of the spindle. Also are you getting any alarms. If so if i had a copy of the alarm and your ladder file i could open it up and see what is causing the alarm.