finger scanner - how to add a 3rd fingerprint?

[GJL65]

I've just bought a used Galaxy Tab S 8.4 SM-T700. Mostly I'm very happy with it, but I'm having trouble with the finger scanner,

When I set it up, it prompted me for two fingers, and it accepts those. But in the Finger Scanner settings it says it "can register up to 3 fingerprints", and I'd like to add my left thumb. But when I go into the "Fingerprint Manager" under the Finger Scanner settings, the only place that seems to be clickable is the line item that says "Fingerprint 1" (I don't know why it doesn't list "Fingerprint 2" also; it does accept both fingerprints). But when I press "Fingerprint 1", nothing happens. How do I add a 3rd fingerprint?

 

[Rukbat]

Is there a + sign somewhere? (I don't have a Samsung at the moment - my Pixel just allows you to add fingerprints.)

 

[B. Diddy]

Can you clear out the fingerprints and start over, this time seeing if the tablet asks you if you want to add a 3rd fingerprint during the process?