Lydy's Anarchist Revival Meeting

I do not have a Mac to play with. However, poking around on the internet suggests the usual things:

-- Switching Dictation off and then on again (which you've probably tried already)

-- Switch Dictation off, restart your Mac, then switch Dictation on again (again, you've probably done this)

-- If you are dictating in Safari, try using a different browser.

Many more suggestions over here, at a source of unknown reliability. At least it gives detailed instructions.