vodacom airtime magically dissapears

I bought 12 rand vodacom airtime 2 days ago, I didn't make use of it in any shape and form. I wanted to make use of the airtime and I checked my balance as my airtime dissapearing into thin air has occured before. My balance currently is R2.99 unless magic occurs once again and decreases. It has created such an inconvieneince that I am considering changing networks as this **************** cannot continue, news flash money doesn't grow on trees.