I hade issues with that as well while playing on a controller. After switching to keyboard I realized I needed to be very precise with my aim and hold down the button for it to work. I also had problems with the slow-mo mechanic at the time I think, so it might be a technical issue and different for everyone.
Anyway, switching to keyboard and aiming with the mouse while holding down the button worked for me after some trial and error (probably like 5-15 minutes of trying :/ )