Only default calendar shows

I just noticed that my iPhone is only showing my default Google calendar, not my other calendars (birthdays, holidays). In the iPhone calendar-screen there used to be a button in the top left labeled "calendars" where one could pick which calendars to show. This button is now gone. Any idea why this stopped working?

This was probably a side-effect of the trouble at Google's Calendar API service last week.
Is it working properly now ?

I have this problem, too. Suddenly last week, all my adpoted calendars from friends were gone. They did not come back since then. I tried everything: resyncinc, forcing resync with resetting data (device managment on nuevasync's page). Nothing helped.
Now I changed my phone (Android with direct sync - works great!). There's an issue with some iPhones, I assume.

This was caused by a Google Calendar service bug/operations issue (the Google Calendar service randomly changed the URLs for some calendars).
We deployed code I think yesterday that should work around the problem if it shows up again.

Yes - it works again. I had to reset my calendar data in order to have resynced all my calendars, but it was worth the effort. The only issue now: my calendars are coloured in different colours, now. It will take me some time to learn the new ones, but that's another problem. Thanks, NuevaSync, you for the help and this great service!

I just started sing NuevaSync in hope it would sync all my google calendars, but only my dedault calendar shows up.
Is this a known issue at the time? Anything I can do to fix it?

Check the calendar sync settings (click the 'setup' link next to calendar on the control panel site).
By default only calendars that can be written are enabled for sync, but you can add the others on that setup page.