Highlighting of errors not working

PyCharm has stopped showing with a red curvy underline where a name is unresolved or uninitialised on first reference.  Maybe I accidentally hit a key-combination that turned it off but I can't figure out how to turn it back on.

I checked in Settings ->Code Style -> Inspections but almost everything is turned on.  I see "Unbound local variable" but this doesn't appear to make any difference.

Any hints?