Numbers Quick Filter not working

Make sure you do not have any other filters set up. When you create a quick filter, it will turn on all filters. You may have some that were set up previously but filtering was turned off so they were not doing anything. Take a look at the Organize sidebar.

I had the very same issue. If you do have other filter(s) set up, you have to delete them in order for the new filter you want to set up to work. In my case, the previous filter is an ongoing one, i.e. it's one I want on more or less all the time. I had to go in and re-create it. Numbers can be very cumbersome at times, IMHO.

If it's practical you can make a copy of the table to do the quick filter, thus saving the previous elaborate filters.

To quickly copy the table click on the circle at the top-left (to deselect all its cells) then just do a command-c (optionally change sheet) command-v.

Thanks, That was the solution!