Hmm, I'm on macOS too and this is a unknown issue for me. Which version of MM do you run on which macOS version? Are there some mesh errors (non manifolds issues or flipped faces) on your source mesh?
(if possible post a zipped file as an attachment)
Just to explain Reduce and to make sure you do it right:
Reduce is based on a selection. Meaning it only affects the selected area (and as you can do a selection on a single object only it affects the active object only). If you want to reduce a whole object make it the active one by clicking on it. Hit Cmd+A to SelectAll.
Now enter Edit/Reduce. It's a good idea to switch on wireframe rendering by hitting W (this is a toggle key).
The slider values for the different modes (ReduceTarget) may be confusing:
On Percentage-mode max reduction happens with Percentage at 100% (which is righthand slider's range)
On TriangleBudget max reduction happens with TriCount at 3 (which is lefthand slider's range)
On MaxDeviation-mode max reduction happens with MaxDeviation at it's righthand limit.
Anyhow as soon as you pull an active slider you should see a change on the wireframe in the selected area if there are faces which can be reduced . If there's a partial selection only, a certain triangle number is needed to keep the connection to the edges of the selection border. At extreme: If you selected a single or a few faces only MM might not be able to collapse these without causing a crack in the mesh.
Also if there are many small face groups (each only a few faces) on the mesh and PreserveGroupBoundaries is enabled this might forbid a reduction.
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Besides Reduce you can also do a reduction in certain modes of Remesh (all but LinearSubdivision):
On RelativeDensity and AdaptiveDensity use a negative Density percentage.
On TargetEdgeLength set a higher EdgeLength.
Against Reduce this way allows e.g. to SmoothGroupBoundaries and to PreserveSharpEdges
