Brush Masking Overlay not showing up

Leaving aside this possible other issue of local adjustments failing to display: one common reason why people struggle with brushed masking (specifically, as opposed to other kinds of masking) is IF their brush Flow and/or Density is set very low, so that there is no apparent effect from 'painting' with that brush. [If it acts like a paintbrush that has been dipped in such a dilute mixture, that it picks up almost no pigment.] This may have been done inadvertently, for example by pressing number keys (1 = 10%, 2 =20% and so on, 0 = 100%). It is a separate matter, than how intense or muted that pigment may itself be (how strong are the adjustments that this mask is to apply). 

Overlay settings are I believe held in common by all the different kinds of masking therefore if only brushing raises an issue, brush settings is where the finger of blame should probably be pointed.

Brushstrokes may be having no actual effect on the image, for example if they are painting in a subtraction from masking, in a part of the image where there is no selection present to be subtracted from. But in that case you should still be able to view these brushstrokes via the Overlay while that mask 'sublayer' is active.

I am also encountering this issue. Overlay does not show up, and any minipulation I do to the mask only shows up in the navigator window. I can see the reslts also under library, but not in the develope tab. I've restarted lightroom and my machine, but no luck yet. 

Adding to what @dj_paige said, you can also click on the color selector & check the transparency of the color overlay.

If that does not help, reset Lightroom Classic following the second (manual) method from this article: https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

Let me know how it goes. Thanks!
Sameer K

I have 'Show Overlay' selected and have tried every other suggested approach and still nothing shows up.

We don't really know what you have tried. But my first thought is that you have set either Flow or Density to zero, and that's why the brushing overlay never shows up.