Search 'Hangs' While Trying to Assign Transactions to a Batch
This can happen for many reason 


check for any error message in Civicrm Log file
check for any error message in PHP error log file


If you haven't got any error message then 


your MySQL DB is old version and you need to upgrade to latest 
 Because some SQL which has been written in MySQL 5.6 is not been supported in Mysql 5.1 - which will not return any value and will timeout 
also check for your Php Version too