By default, kwin assings meta+= as zoom-in and meta+- as zoom-out. For some reason, I can't use the '-' from the numpad on my desktop keyboard, only from the one next to backspace (when using US layout). If I try to reassign the shortcut as meta+numpad-, the same happens: the reassign is accepted, but then only one of the keys with '-' works. I normally use pt-pt layout, but as I said above, this also happens with us. (In pt-pt, the -_ key is where the /? key is on the us keyboard).
*** This bug has been marked as a duplicate of bug 56871 ***