ics attachments received attached to email message do not load in iCal

I receive iCal invites as an attachment in email messages on Apple Mail. I used to be able to click on the icon for the attachment and the event would automatically load into my iCal. Now, suddenly, it does not. It acts like it is loading but does not appear on iCal. However, when I drag the icon to the desktop and then drag it into iCal it loads no problem. Any suggestions?