I have been slotted for ALC 3 times and they keep cancelling my school dates (''overbooked''). Can I call the IG?

Not sure this is an IG complaint if the school is cancelling your slot.

I don't believe its an IG complaint. If there are units spinning up for deployment they could be front loaded so they can fill the squad leader role for folks PCSing or ETSing. Also folks who were promoted to E6 while deployed would be pushed to the front so they have the schooling required by their rank although they may have stopped promoting before schools since I got out.

This could be due to the backlog the Army is still trying to overcome due to the STEP program. I'd recommend you speak with your Training NCO to confirm this is the reason.