Photoshop paints on the image instead of its layer mask

Choose the mask thumbnail first

Save a separate working copy while diagnosing the problem. Keep the original layered document: flattening or applying a mask is unnecessary for this check.

Make one small, reversible stroke

Find the layer that contains the object you want to hide. Select its mask thumbnail, choose a painting tool, and make a small black stroke over a recognizable edge. According to Adobe's mask editing guide, white reveals, black conceals, and gray produces partial visibility.

Look at the thumbnail as well as the canvas. The stroke should change the mask's grayscale pattern. It should not leave a black painted mark in the image itself. Undo this diagnostic stroke after inspecting it, then make the intended correction.

For example, hiding part of a logo should reveal what is underneath that layer. It will not necessarily reveal a transparency checkerboard if another visible layer occupies the same place.

The mask changes, but the canvas does not

A mask can remain attached while its effect is disabled. With the affected layer selected, check Layer > Layer Mask. If Enable is available, choose it and compare the same edge again. Adobe also documents Shift-clicking the mask thumbnail as a toggle in its enable/disable guide.

Avoid repeatedly toggling without comparing the result—you can finish with the mask disabled again. If enabling it reveals the expected cutout, keep it enabled and continue editing that mask.

Check which layer is actually visible

If the mask is enabled and its pattern changes, inspect the layer stack. An identical visible copy above the masked layer can make the canvas appear unchanged. Temporarily hide that covering copy, inspect the edge, and restore visibility if it belongs in the composition. Do not delete layers to identify them.

If the problem persists, use the support links in Adobe's mask editing guide with a cropped Layers-panel view and your Photoshop version. For a Smart Object editing warning, see ways to edit or mask a Smart Object. Rasterizing the artwork is not the first step for a mask-selection problem.