The most common reason behind this problem is the fact that device was not properly provisioned by mobile carrier for usage with BES. However, there might be a number of other problems.
To make precise determination, pllease post snippet of agent log file (it has 'MAGT' component in the name) covering period of time when device was in the process of activation. Also in case if you do not feel comfortable to post the log, please feel free to send it over to
support@zimbra.com or
eugene@zimbra.com