air time disappear all the the time

eventually this is a daily thing now i buy airtime and data, airtime will disappear without calling anyone but the data will be in the same way i left even though some will be chaos..maybe i will be in face book or browsing but i don't understand am now afraid of buying airtime yesterday i bought R30 airtime and used R10 to buy data all of the sudden am left with R10 and MTN send free R60 airtime which is i get it when i used R6 on the my airtime. please fix your system...two weeks back i call on Sunday after it happens other consultant call Matumelo told me they will NOT refund my airtime because i open lots of thing on my phone which is not right and which means i will only buy airtime that i going to used immediately. Sucks that way.please check you system CZ i want all my airtime you busy chowing. am sick and tired of the service and how you treat us as the your customers...and am tired of transferring my airtime to people to save