I'm going to take a stab in the dark and suggest that your configuration is probably resulting in no highlighting.

You can check this with the command `:verbose highlight SpellBad`: if the `xxx` at the start of the line is not highlighted, then badly spelled words won't be highlighted, either.

If this is the problem, then spelling *commands* should still work correctly: you should still be able to jump to the next and previous error with `]s` and `[s`, etc.

You can fix this by setting the highlighting yourself. You can simply add `:highlight` commands to your `vimrc`, but a better way is probably to use a `ColorScheme` autocommand. e.g.:

    augroup my_colours
      autocmd!
      autocmd ColorScheme adrian hi SpellBad cterm=reverse
    augroup END

Put this before the `colorscheme` line in your vimrc.

See `:help :highlight` and `:help ColorScheme` for more details.