SQL Server Mirroring disconnected for 1 of 5 mirroried DBs after initial setup completes
You may not have enough available threads.
See how many are available with sys.dm_os_sys_info.  See how many are being used with sys.dm_os_schedulers (sum of rows in column current_workers_count)

Database mirroring has the following requirements for worker threads
Principal server: 1 global thread and 2 threads per each of the mirrored databases.
Mirror server: 1 global thread, 2 threads per mirrored databases and one additional thread for each mirrored database for every 4 processor cores. 
On a 4 CPU server it requires 1 global thread+ (2+ 1) threads /mirrored db.
On a 6-8 CPU server it requires 1 global thread +(2+2) threads /mirrored db. 
https://support.microsoft.com/en-us/kb/2001270

You may not have enough RAM.  You may not have enough network bandwidth. Your disk subsystem may be overwhelmed. Your servers may have too heavy of a load to support another mirrored database.