Unable to add another sample to existing sample on pad

Hi
I’m trying to layer 2 kick drums.
Pad C1 has a kick sample on it; I was expecting to be able to simply find another kick and drop it on there so that there would be two layered samples, both playing at the same time. It’s as simple as that right?
But all I can do is replace the existing sample.
I cannot see anything that might indicate I am in some kind of special “replacement only” mode.
I’ve check the help at Assigning Samples to Pads (btw I am using GA not GA SE).

Not really much I can show because you can’t exactly take a screenshot of this: when I drag the sample over the pad only the middle row (square arrow square) lights up; the others are dark.

GA 5.0.30.233 Cubase 11.0.40.446 any help appreciated. Thanks.

Actually I’ve figured it out (I think) - it’s because what I’m trying to drag onto the pad is not a sample; it’s something more than a sample (it has an icon with what looks like layers). It does only contain one sample but it’s not “just” a sample. I can drag “just” a sample onto the pad and it does layer. So seems very likely this is expected behaviour.