@LeKill3rFou
The only cyberware mod that I believe affects finishers is the Target Analysis mod, so if you don't have that equipped, any katana should do its finisher pretty frequently. There's also a Pax mod that changes your weapon to non-lethal, but I think that mod can only be attached to guns, and I'm under the impression that it only makes that specific weapon non-lethal. I guess you can check to see if you're equipping a gun that has the Pax mod.
The katana finisher is pretty subtle compared to the mantis blades. The katana finisher animation has a slight pause, then a quick slash that cuts off your opponent's head. Overall, it's very brief, unlike the mantis blades finisher animation that lasts a few seconds.
To troubleshoot, you could switch to the mantis blades to see if you get the finisher animation with that weapon.