Airtime disappearing

Hello Peter, I am having a problem with my mtn number, February this year I received a sms of subscription to my number. Because I was not aware of the subscription I unsubscribed and it was a success. But I am still experiencing the same problem. Two weeks ago I recharged with R10 airtime with an aim to buy R10 minutes for 7 days, but it was unsuccessful because I had insufficient funds. The same thing happen this morning I recharged with R10 with the aim of buying 100minutes for seven days. That transaction was not a success because I had insufficient funds. When I am calling MTN I don't get any assistance because they cannot pickup what is happening to my airtime. This had happened on numerous occasions. Every time I recharge there is potion of my airtime that disappears into thin air.