When using selection or layer my Brush tool used to show the layer underneath...
This suggests to me that at some time you have had a layer MASK on the layer.
When you paint with BLACK it reveals the layer underneath...
When you paint with WHITE it restores the layer with the mask.
...now it paints black or white.
This suggests that now you DO NOT have a layer mask OR you do but the Layer Pixels is 'active' and not the Layer mask.
In relation to image you posted you DO NOT have a layer mask on the top layer.....thats the problem....so....
...Select the TOP layer...
...add a layer mask to the TOP layer via the 'Add Layer Mask' icon at the bottom of the layers panel.
Your Foreground and Background colours will change to Black and White automatically every time the MASK is 'active' (click on it).
This happens because you can only paint with greyscale on a MASK...ie, Black, White and everything in-between.
Just REMEMBER...
Select EITHER the LEFT Thumbnail to paint on the PIXELS, or the RIGHT Thumbnail to paint on the MASK.
Regards.
MrTom.