DWP say tribunal papers were not received

DWP are saying they didn't receive the tribunal letter sent by the tribunal service in January, tribunal service have now sent the letter again and they have until 14th may to respond,that will be 14 weeks since first letter was supposed if been sent. I only received a letter with tribunal reference,as did my representative. is it possible that the tribunal service messed up and didn't send the letters out in the first place, I've asked them,but cannot get a straight answer from them. What should I be doing now, neither tribunal service or DWP won't give an honest answer