How are you trying to sync them? Via iTunes or via iCloud? What version of OS X and what version of iOS?
There's really only two ways to sync calendars, and basically it's as simple as checking a box.
For syncing with iTunes, see here: http://support.apple.com/kb/HT1296
For syncing with iCloud, see here: http://support.apple.com/kb/TS3999
I prefer to sync with Icloud an get rid of every unnecessary calendars. But i find that the support-pages on Apple are to complicated to get a quick fix for that. So Icloud calendars will do for me.
I donnot want to spend to much time to find out. And I notice that a lot of people got the same problem. We just want an easy way out.
I find that Itunes is a little bit outdated here.
ICloud is probably what people are using.
It is a bit cumbersome to first go into Itunes to sync. As long as I am most of the time on the same Wifi as my phone, this is a much easier solution.
I guess I found out for myself. Do not have any other calendars than Icloud on your machine. Otherwise they won't sync.
The problem arised when I startet to use Icloud to sync instead of Itunes.
The transition was far from smooth.
So after many missed appointments because of lack of syncronistation I finally understood why.
Well. What about a smart an easy user introduction from Apple on how to use Icloud in an efficent way.
I think many can say thay have I-mist instead instead of I-cloud for a certain time before they understand how to use Icloud.