Make sure you have the most recent version of Fantastical.
3.0 was completely broken on my Apple Watch.
It is still buggy now but works most of the time for me.
I believe, if you have v3 on the phone, you also need to create a Flexibits account to get watch syncing to work. Not sure if you also need to set up a direct connection between the calendar and Flexibits opposed to using the local calendar which previously worked.