You should check the following things first:
a) the handheld being in poor network coverage
b) a bad IPPPP service book on the handheld
c) the MDS service not running
d) MDS not being enabled for the BES OR for the specified userxxx8217;s account (right click the BES and the user and select xxx8220;enable Mobile Data Servicexxx8221;)
e) You need to make sure that port 3200 is both LISTENING and ESTABLISHED with the bmds.exe. The best way to check this is to use TCPView. You can download it from: http://www.sysinternals.com/Utilities/TcpView.html
Also make sure you have the proper version of Java installed for the Version of BlackBerry Enterprise Server that you are running.
Hope this helps,