F16-F19 not working for Spaces

I want to use F16-F19 to switch to space 1-4 respectively. Only F16 works for Space 1 and F17-F19 don't work. Anyone know why? I searched and found a 3rd party app mentioned called Spark but I'm not interested in having yet another app running - I think this should just work.

I configured it for Ctrl-1-4 for now and they work, but can imagine how nice it would be for the unused F16-19 keys to work for this...