Disappeared airtime

I bought airtime last night via cell phone banking, sent 1 sms , when i'm making the call it says i have no airtime and when i check balance is .00 . I called mtn the lady assisted me said airtime is not updated i should wait for 2hours, i called again this morning the other lady said my airtime is not updated yet i should wait for 24 hours. I could not make the calls i bought that airtime for and there's nothing they can do as per consultant. How on earth one can buy airtime and use it after 24 hours??