ZBrushCentral

Can't add to hidden parts

Hello,
I am new to zbrush.
I am trying to remove parts of my mesh using ctrl + shift to select areas to be hidden.
When I try to add another area to hide I :

  1. press ctrl+ shift and click.
  2. release the keys and press alt.
  3. click and drag with the red selection area,
    Only the last selected part is shown instead of adding to the hidden parts.

I have also tried selecting and letting go of the shift key but this does the same thing.
Can someone help please