transfers

Last year I've bought a airtime bundle that will give me R75 airtime per month over 12 months ......I do receive my airtime on time but if I want to help a friend/family with some data/airtime I'm refused to transfer the amount that I want to...it just shows that I don't have enough credit and need to recharge...my question is if I have airtime why can't I use it as I see fit ?... I don't believe that a outomated system can be so dumb as not to be set correctly to recognise that my account has credit availible to use ...pleeeeeeese help me