Due to latency between your client and the server, is it possible that you activate it, it shows up in your Sys.Info window, but then you still get hit and die? This particular situation should be considered "Normal", even if isn't good =)
Try testing it with something that can't kill you in a couple of hits (Yubos are good). Get something to attack you, activate Invuln, and see if it ever kicks in. You should see messages in Sys.Info saying that "Whatever hits you but it has no effect" or something like that. If after 5-10 seconds it STILL hasn't taken effect, that's definitely unusual.
Try testing it with something that can't kill you in a couple of hits (Yubos are good). Get something to attack you, activate Invuln, and see if it ever kicks in. You should see messages in Sys.Info saying that "Whatever hits you but it has no effect" or something like that. If after 5-10 seconds it STILL hasn't taken effect, that's definitely unusual.