Could be the latest, greated MacOS issue 
or a best guess. Whatever you should check the selection mode
The fuzzy select tool is in addition mode, see the little + sign next to the cursor.
If there is already a larger selection in place then adding to the selection does nothing.
either
Turn all existing selections off. Select -> None then use the fuzzy select in addition mode
or
Use the fuzzy select tool in replace mode, (the first, left side, mode icon)
30 second demo: https://i.imgur.com/NJaUEH6.mp4
or a best guess. Whatever you should check the selection modeThe fuzzy select tool is in addition mode, see the little + sign next to the cursor.
If there is already a larger selection in place then adding to the selection does nothing.
either
Turn all existing selections off. Select -> None then use the fuzzy select in addition mode
or
Use the fuzzy select tool in replace mode, (the first, left side, mode icon)
30 second demo: https://i.imgur.com/NJaUEH6.mp4