Decals will not appear on any surfaces in my model [help]

I’m having trouble making my decals appear on any of the surfaces in my model. I’ve tried changing the material/textures of the surfaces. I’ve also tried using different image formats.

Its quite frustrating as the tutorials I’ve seen make it seem like a very straightforward process. I can’t tell if its a bug or myself making a simple mistake.

All help will be gratefully received.
Many thanks!

Hello - that looks like it ought to work - is the viewport in Rendered display mode?

-Pascal