Why mtn when I recharge they keep taking my airtime

I having a problem with mtn they keep taking my AIRTYM because they say I have an outstanding balance of extra time I recharged with close to 120 AIRTYM this past 2days but they keep taking my AIRTYM and my outstanding balance was 50 but they just can't stop wen I recharge they take my AIRTYM called their agent spoke to 3 of them they told me it was a technical problem they will resolve i it I asked if is like that why do they keep taking my airtime the answer was they don't know. it pains me because I couldn't call my mother to wish her mother s day mtn is the worst network service will never advice anyone to use it with their *********** agents