how clean?

How clean does the silicone groove have to be in order to get the silicone to stick?

Depends what’s making it ‘dirty’. Generally speaking, the cleaner the better.

Take cotton swab. Dip in alcohol. Make a couple circles in the recess. Repeat on the other one. That’s what I do. And of course, if you can actually see something in there, do it again. And if it still doesn’t stick, then you can, although I don’t, use a nail or needle to carve/scratch into the recess, to rough it up and give the silicone more surface area.