Check how many devices are allowed in throttling policy...
Get-ThrottlingPolicy | select name, EASmaxdevices
Here are the results of that command. I think I am missing something.
[PS] C:\Windows\system32>Get-ThrottlingPolicy | select name, EASmaxdevices
Name
----
GlobalThrottlingPolicy_cc5d0b3f-03e8-4988-b758-782e55d7054e
SymantecEWSRestoreThrottlingPolicyExchange2013
The reason for this is Exchange 2010 SP 1 introduced a new feature called "Throttling".
To find current limits:
Get-ThrottlingPolicy | select name, EASMaxDevices, EASMaxConcurrency
(you will get all ThrottlingPolicies available in Exchange environment and how many mobile devices can you connect to the mailbox, and also using EASMaxConcurrency you will see how many connections in one time can be accepted to the mailbox.)
To change the number to 30 (just a random number) run the following cmdlet:
Set-ThrottlingPolicy EASMaxDevices 30
Identity DefaultThrottlingPolicy_hex_string
To find the hex_String do a "Get-ThrottlingPolicy | fl"
For a user to see current devices tied to their account go in via OWA > Settings (upper left cog) > Options > Phone. Check ow many devices are here and when the last one was used.