You don't give any specifics, so it's rather hard to address your
question, but here's a few thoughts:
1) Some keyboard shortcuts are hard-coded. For instance, you can't
remove CMD-SHIFT-P from the Project Gallery command using the remove
button. However, if you assign that shortcut to another command, it will
no longer appear under, nor work for, Project Gallery. If you then
delete it from the other command, it reappears under Project Gallery.
2) Even when a custom/removable keyboard shortcut is assigned to a
command, the Remove button is disabled until you select a shortcut.
3) The Add button is always disabled until you enter a shortcut in the
textbox.
4) Some keyboard shortcuts are mandated by the System and can't be
assigned, e.g., CMD-H, which hides the application.
5) Some keyboard shortcuts are hijacked by the system, even if you
assign them in XL. Go to System Preferences/Keyboard and Mouse/Keyboard
Shortcuts. If a shortcut is assigned and active in this pane, XL will
never see the keypress. You can reassign or disable shortcuts in this
pane to restore XL's shortcuts.
6) A few commands can't have shortcuts assigned from the GUI. Many WinXL
users are used to using F2 to edit a cell, for example. You can't access
an EditCell command directly, but you can use a similar workaround using
the EditFormula control - see
http://www.mcgimpsey.com/macoffice/excel/f2.html
If these don't address your shortcut issues, post back with specific
questions/examples.