Data bundle expires before the 30day

This is the second time I've experienced this. In December I bought me a 50gig Mo'Town bundle which is meant to last me 30 days , it expired before the 30 days , before the 25th day I had to buy again but decided to buy on the 26th of Jan for Feb so that I at least am able to replace it on the 25th , I still had data about 7gig left , now its the 23rd and it has expired , why put in a 30day to it and then the data expires before the 30days , I know I did not run out of data even in Jan i did not run out of data . The Mo'Nice works fine if it is for a week it will last for the week .