MySQL Master not sending events to another Master
This could be one of two things


Slave I/O Thread is encountering Network Issues
Slave I/O is Playing Possum (pretending to be dead)


Derek Downey and I have written posts about the flaky behavior of the Slave's I/O Thread


Oct 17, 2011 : Master port is disabled from Public IP, MySQL log is not getting refreshed
Sep 30, 2011 : Can we capture only Slave_IO_Running in SHOW SLAVE STATUS in MySQL)


According to the Bug Reports on this phenomenon, it was supposedly fixed.

See (Bug #30703 and Bug #51089)

If this bug is happening to you, someone missed a patch making MySQL 5.6.23.

I have three suggestions


Drop back to MySQL 5.6.22 or 5.6.21 or last version of MySQL 5.6 where replication was working
Use MySQL 5.6's heartbeat option when running CHANGE MASTER TO.
Implement semisynchronous replication


As for the Network


Check Network For Dropped Packets
Check the NIC Card
Use separate NIC card for Replication Traffic


Other that what I have just said, sorry I don't have a silver bullet for this one :-(