You need to look and make sure the head isn't damaged where the copper seals. If that looks good, make sure the injector bore is clear of any debris and clean out the hold down bolt hole really good with some brake parts cleaner and an air blower to make sure there isn't anything in the hole stopping the bolt from torquing down properly. Once that is all done, reinstall the injector with a new copper washer, new bolt, and new hold down. I like to use a light coating of ARP lube on the threads of the bolt and under the head then torque to spec.