Well, if you write to them and tell them there was no ticket there... it must happen all the time, it happened to my mum once. Then you get the discounted rate again. Still means you have to pay.
However, examine the line where you were caught very carefully. There are strict guidelines on these. Simple things such as a lack of a T-bar on the end of the line, or a few feet where the line is not complete from end to end (i.e. a few feet of paint missing somewhere on the line). There may also be regs on the signage informing you of the hours. People have had parking tickets nullified this way.