Mail not consistently adding events to Calendar
When I click "Mail Attachment.ics" files in messages in Mail, sometimes it creates an event in Calendar for me to accept, sometimes it doesn't, and sometimes it doesn't the first time but it does if I try clicking it again a minute later. What can I do to make this process work smoothly? At the moment I often resort to processing calendar invites on my iPhone or via gmail.com.
iMac, OS X Mountain Lion (10.8.2)