Change your keyboard? Q always works fine here. It just stops working because you're probably spamming it and triggering the "dead Q" timer 
This is a nice tip actually.
If you see someone walk past you, it doesn't necessarily mean that they have spotted you so react accordingly.
For example, sometimes I just spot the guy who runs past and let my squad kill so I can pick off the medic which tends to go revive or start firing on people behind the guy who just died.
So many things come down to game sense....

Always spot, especially if you are a sniper with the 12x scope. You can often rack up 100's of points just by getting in a good position and spotting people.
Also, Wait for the right to time shoot! So many times i have seen someone before they have seen me and just panicked and shot at them. this normally ends up withe me missing horribly and them turning round and killing me. So, before you let some rounds of always make sure you are in a position to kill them first.
I used to be terrible at just reacting straight away if i saw movement but I have now learnt to assess the situation a bit better. I have found this to be very important in thsi game
This is a nice tip actually.
If you see someone walk past you, it doesn't necessarily mean that they have spotted you so react accordingly.
For example, sometimes I just spot the guy who runs past and let my squad kill so I can pick off the medic which tends to go revive or start firing on people behind the guy who just died.
So many things come down to game sense....


Grrr!!!!