Outlook calendar updates don't update on iCal

I know others have this problem but I've never seen a solution.

If someone sends me an invite from Outlook, I get the .ics mail attachment and it goes into my iCal OK. If they send an update (to change the time, for example), it comes as a .ics attachment but it doesn't update on the calendar. It causes me a lot of issues and I'd love to find a solution. Anyone?