Very often turret index faults are due to contamination on the proximity sensors or microswitches. There are two; one detects the position for tool 1 and the other counts the number of tool spaces. They are not easy to see but if you remove all the tools, do a tool change using 5% Rapid and hit Estop while the spindle is lifting you can see inside the changer. There is a sensor on the track for the shuttle so you can see what they look like and you will see the two inside. If they have a chip stuck on them that will be your problem. If they are microswitches they may be gummed up with coolant residue.