[Imaginos]

If the connection sometimes works, it seems you have the email accounts configured correctly. Did you verify in connection manager that Exchange Push is active?

Exchange server and IIS is operating ok and licensed for the necessary # of concurrent connections? If using exchange 2007 server, are there policies in effect to limit the usage hours of activesync? Are there any other handhelds besides the Moguls that EAS to the server you can use to verify there's no activesync issues on the server side?
Do all the handhelds have a good dataplan and air coverage for solid internet connectivity? When the OTA sync fails, are they also unable to surf the web?

Sorry to pepper you with all this. Trying to narrow it down. My Mogul is a solid EAS performer, so I can't imagine your 10 phones all being bad. It's got to be something else common to them all.