Airtime vanishing

My airtime just keeps on vanishing , every time I call MTN I just get stories and they don't return my airtime and it stopped for a while after calling to report but now it started again .<br> On the 27/02/2015 I recharged R12 and bought data for R5 and the rest of the remaining amount disappeared and yesterday I loaded another R12 airtime and without me using it they took my R3 .Please refund my airtime .