Photoshop CC, Windows 7, keyboard shortcuts dont work

For retouching, I mostly use the Sponge and Burn tools. I modified the keyboard shortcuts to make sure that the Sponge is the only tool associated with the S key, and same thing for Burn and the B key.

My problem is that when I press S to select the Sponge, work with it a bit, then type B to select the Burn tool, it does not get selected, and very often Photoshop tries to "Select Subject" (although that menu item is not associated to any keyboard shortcut at all). Re-assigning a shortcut to the Burn tool (for example, Z, which is not associated with any other tool or function on my configuration) does not make it work better. My presses on the Z key just remain ignored, and sometimes the Slect Subject function launches.

Any idea how to cure this? It gets painful having to use the mouse to select the tools I need instead of their shortcuts.

PC running Windows 7 up to date and latest version of Photoshop CC.