Rarely an achievement will fail to trigger as the game doesn't receive the right response back from the Steam backend in time. Sadly when this happens you will need to work for the achievement again. We have no way to directly touch your account or give you achievements or stats.
If the achievement has failed in multiple attempts on the same server (or solo game) the issue likely that the server or client is running a mutator that is not whitelisted. When running a non whitelisted mutator perks will not progress and achievements will not unlock. Check to make sure that no mutators are being run.
