Is your Exchange 2010 server a server using migrated accounts?
If so, please read my article about the possible reason why Activesync does not work:
http://www.experts-exchange.com/articles/Software/Server_Software/Email_Servers/Exchange/Activesync-Working-But-Only-For-Some-Users-On-Exchange-2007-2010.html
Have you also installed a 3rd party SSL (SAN / UCC) certificate?