Electricity money deduction

My water bill and my Electricity bill is linked. Last night I bought Electricity knowinh my water bill of R480.64 should go off first. I first bought R600 then it didn't go through. Then it said I should try again later. However, the 600 still went off. I tried again with a R800 then it only paid my water bill and with the remainer it gave electricity as it should. So I was want to know where my 600 went? This is the 2 time it happened.Last year May it happend to 2 * R500. Where it doesn't pay the bill nor does it give units. Please look into that 500 * 2 and yesterday's R600. The money doesn't show on the water account or anything it just goes somewhere.