My selection does nothing

You wired a selection into a node and the shape came out exactly as it was. There are four common causes and they look identical from the outside.

The selection is picking nothing#

Your threshold may be past everything the field produces. Noise mostly lands between nought and one, so a threshold of two selects no face at all, and the node dutifully changes nothing.

Drag the threshold and watch. If the result snaps from nothing to everything without passing through a middle, the threshold is in the wrong range rather than slightly off.

The socket ignores per-face values#

Only some inputs read a different value for each face. Wire one into anything else and the app will tell you: the connection is refused rather than accepted quietly, because a wire that looks connected and does nothing is worse than a wire that will not connect.

If that happens, you want the node's actual selection input, not a numeric setting on it. Thickness and Depth are single numbers for the whole node; Selection is the one that varies per face.

The node only asks yes or no#

Delete Geometry, Grow Selection and Extrude Along Curve read whether the value crossed the line, not the value itself. Feed them a beautifully varied field and every selected face still gets identical treatment.

For uneven results use a node that scales by the value instead: Inset, Set Position, Extrude Tiles, Bevel or Color Ramp.

The faces are not what you think#

If no Set Faces has grouped anything into faces, the selection is landing on triangles, so a change covers half a tile and reads as nothing happening in particular.

And a grouping goes stale the moment the shape's topology changes. If you grouped faces, then thickened or cut the shape, group again afterwards.