Why are some iCloud services not syncing with my Mac?

Question

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

Answer 1

For photo stream, if the source of the photos is an iOS device, be sure the photo app used to take the picture has been turned off, not just sleeping.
Have you rebooted the computer?  The router?