I have been having child support deducted from my paycheck but my ex says they are not getting the full amount.

Go to your HR department first.  Are you sure your paycheck shows the full amount is being deducted?  There must be a mistake.  SInce you don't know who made the mistake, you need to assume the person you are speaking to was not the one who made the mistake.  By checking at each level, i.e., employer, payroll service (if any), state, and ex, you should be able to learn who is mistaken, but right now, remember, while someone is in error, you do not know who it is.  So do not get confrontational with any of them, but be persistent.  If the error is on your end of the equation (eg., your employer), you will still owe the money, and back child support will likely add 20% to what you are paying until you get current again. So you need to do some digging, and the sooner the better, as this problem will only get worse with time.  Good luck.