If you are sure that your samples should amplify (you've carried out the same experiment with a newer kit and everything works fine), then, most likely, there is a problem with the kit. However, as Laurence mentioned, some kits work fine after the expiration dates when they have been stored properly. If your kit has already expired and it has been exposed to several freezing-thawing cycles, there's a big chance that your kit´s efficiency might be compromised.