Sometimes I am unable to use my support bonus no matter how much I press the damned button. Yeah, it's a custom button for me, Left Alt, very easy to use, but rarely it happens that even if my support bonus is available, nothing happens, even when I respawn.

Same issue. Its most frequently EMP for me.

Same issue. Its most frequently EMP for me.
Same, I had EMP available and it did not work.

Could be a re-binding issue/bug. Did you try resetting it to it's default?