Can't buy Just 4 You bundle

I have been trying since yesterday to add a 1GB Just 4 You data bundle. Each time I do that it gives me an error. But when I buy a 1GB voucher for 1 day it works. I can't pay R29 every day and I am not prepared to pay over R100 for 1GB that won't last me 30 days or more. I usually do the 1GB for 7days thing. This is not right.