If you are using Google Sync service to sync your calendar to iPhone:

http://www.google.com/mobile/sync/

You may find that only the first calendar will be sync to iPhone. If you want to sync more than one calendar:

1. Visit http://m.google.com/sync?hl=en ,the reason add "hl=en" at the end of the link is that it seems there is bug in the web that it only works in English language. It also didn't work in http://m.google.com.hk

2. Select your device, and check the calendar that you want to sync. Save and wolla!