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 nothing was grouped 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.