Why are some iCloud services not syncing with my Mac?

iCloud seems to be syncing just fine between my iPhone 4S (AT&T 4G) (iOS 5.1.1) and my iPad 2 (Wi-Fi + 3G) (iOS 5.1.1), but not with my MacBook Air (Late 2010) (Mountian Lion 10.8.1). Here's a list of iCould services that I've noticed are and are not working:

• iCloud mail (@me.com) and Gmail syncs and updates on my computer sometimes. (I get that these might be seperate issues. I think I need to change the "port" on one or both – suggestions?).
• Exchange email syncs and updates on my computer most of the time.
• Notes syncs and updates on my computer never.
• Photo Stream has stopped updating and syncing on my computer altogether (worked for the first few weeks and not for the past few weeks).
• iMessage synced with my computer and phone before Mountian Lion released (that is, all incomming iMessages sent to my Apple ID would be recieved at my phone and computer), but I don't think it's worked since.
• iCal, Contacts, and Reminders seem to update and sync on my computer all of the time.

Because they're all iCloud services I want to think there is a one common problem – which is why I've created only one thread. But because they are not common to all services, I'm guessing it might be a case by case basis.

Of course I should state all iCloud services are turned on for all devices and computer, and I am using the same Apple ID for them all. Also, all devices/computer are connected to internet via Wi-Fi/4G pretty much all the time. Finally, it's possible there are other issues I've missed—although it seems unlikely since I use them all, all the time)—I think this is a pretty good representation of my problems.

Thanks for your help!!

Sarah