You do not have funds, yet my balance says differently !

Hi, I've had this issue before and it thought it was just a mistake of some sort and left it the first time. Now I am getting very irritated. I received some promotional codes for USSD based competition. Which you need airtime for. I load R15 on and submit the USSD code and low and behold I get the "you do not have fund for this transaction" or something like that. Why cant I use the airtime I paid for, for something as stupid as entering a competition?

Please sort this out.