problems syncing ical on the iMac

I use mobile me to keep all data synchronized however recently I am having the following problem with the calendar.

The calendar on my iphone and on the MobileMe web synchronies perfectly, any addition that I make to iCal on the iMac synchronizes perfectly as well, however any changes made on the web or on the iphone do not synchronize back to the iCal on the mac.

Please help,

Thanks

Antony