Appointments+ not syncing with google calendar

Install seemed straightforward enough but.... I can't get the last bit of it to work. The synchronisation with google calendar.

Without google cal sync, I can't use the plugin. I have other appts on my gcal and don't want people on the site to choose a time I'm already booked.

The appts get created but not added to gcal.

When I clicked to test the integration, I did not get an error.

When I import/ export manually to gcal, I get a message saying "no future appts found to add to your calendar".

Any idea? What are common things that go wrong?