CSA halved my payments without warning

My ex has his payments deducted by his employer and this month without warning they halved my payment  and i recieved £218 less than i should of when i contacted csa asking why this was they said they could offer me no explaination as to why ths was and said until it has happened for three consecutive months they would not even investigate why this was ?? so this means arrears will build up on the case in the meantime .. i have had endless problems with them over past three years and their timescales and even contacted my local mp to get involved before because of the way i was treated they have no ownership of indiv cases and are not sympathetic on the end of the phone.. which too has cost me lots of money over the years due to the cost of ringing 0845 numbers the csa are meant to make life easier when couples split and save arguments over money ensuring parents pay for their children but they have major flaws in doing their job properly ..