I don't know what has changed but when I apply a layer mask to a layer to remove part of that layer. I select the brush and switch to black and it doesn't paint out all the background. What am I doing wrong?

It is difficult to understand what you are doing as a whole.

"Apply layer mask" really means to apply the effect of an existing mask after which the mask is removed.

In case you mean you create a layer mask and paint with black on it to make the black area inactive and if for some region it doesn't work as expected check if you have any selection active of if you have a clipping mask or something that may be affecting your brush.

Or if you are doing something else - explain.

When you alt-click or option-click on the layer mask so that you see only the layer mask, what do you see?

Pure blacks and whites? Or a bunch of grays from painting with pressure or painting with gray, etc? Is your brush itself set to normal blend mode? Or something else?

Here are some things to check:

1. Make sure your brush is pure white and pure black by clicking on the Black and White squares right above the 2 color squares in the main menu. This re-sets the painting colors to the default pure B+W colors for you.   If you are not painting with pure black...and are getting grayish tones...then your masking will be incomplete.

2.  I love the suggestion above... alt-click on the mask itself...and your original image will disappear and then you will be able to see the pure black and white mask itself... by itself ONLY.  If your blacks are not pure black... you wont to be able to completely paint things away.

3. There are several other things to check.  Is your brush the first brush on the adobe pallet?  Some brushes have built in presets that make them NOT paint pure 100% black. They may have the Flow control at something other than at 100% flow. Also be sure your brush blending mode on the main top menu is set to NORMAL...and not some other blending mode live overlay or something else.  And make sure your brush opacity is set to 100%on the main top menu or whatever percentage you need.  Sometimes you might have the air-brush icon pressed in... in that case you have to paint over several times to get pure black...make sure the airbrush icon is not pushed in.

4.  Make sure your "masking layer itself" is NOT in some blending mode other than NORMAL and at 100% opacity...otherwise the black will not completely erase the original image.

These are some of the things that may make the black brush NOT be able to totally erase the thumbnail image.

This sometimes happens to me and probably 95% of the time, the problem is that I may have inadvertently created a selection around a different area of the image. So I have now developed a habit of using the deselect keyboard shortcut (command, D) and then checking once again to see if my brush does what is expected when using black or white on a mask once again.

Another one to look out for: as you know, you cannot edit the layer mask with a normal mask present. If you click Ctrl+layer mask thumbnail you create a mask from the layer mask. However, if none of the pixels are made 50% or more visible by the layer mask then the mask outline (marching ants) will not be shown, even though there is now a mask present! Therefore, you may not think there is a mask present when there is.