The issue you are having is the "spawn protection". This occurs immediately after spawning and goes away shortly. One thing that affects your spawn protection is if you move your mouse or joystick (if on console), so it will immediately go away if you spawn and move.
As far as the knife kills, there is a period of time that there is some protection also, and is a bit random. For fixing it, I think this would be a bit difficult and may result in more bugs, so I would like them to keep it how it currently is.
Edit: whoops, Spitfire beat me to the punch lol. Had some tabs open and answer them after looking thru them.
