mail not syncing

Hi,

I have encountered an unusual problem since last week that I can't resolve. Mail on my iMac (running Maverick) has stopped syncing with my iCloud account. It is very intermittent though as sometimes it get new mail as it arrives, other times taking ages to get the mail. What is really odd though is that if new mail arrives and I open it in the web browser account while in work, or on my iPhone or iPad, the mail doesn't sync to the Mail on my iMac.

I have checked all of the account settings which are correct, I have tried rebuilding the mailbox, and I have tried syncing with iCloud manually. I have also tried switching off the mail setting through the iCloud in System preferences and rebooting before turning it back on, but nothing seems to be working

Grateful for any advice!