Why a shortcut may not fire

Check these first:

• The focus is in a text field, a number field or a dropdown.
• A modal dialog is open — every shortcut is suppressed while one is.
• You are in Diagram mode, where everything below undo/redo is skipped because the diagram owns Delete and Backspace.
• A report sheet is up, which owns the keyboard the same way — see above.
• An annotation tool is armed, in which case the tool owns the keyboard.
• You are on the login or projects screen, which has no viewport and no listener.

The blend-array panel is a deliberate exception: it is modeless, so shortcuts keep working while it is open.