The master never return to semisynchronous replication (1 reply)

I have configured a master and a slave with semi-sync mode.

In the document "If a timeout occurs without any slave having acknowledged the transaction, the master reverts to asynchronous replication. When at least one semisynchronous slave catches up, the master returns to semisynchronous replication."

There is a test scenario.
1. Cut down the network of slave's machine by "iptables".
Reject the access by port "3306"
2. Insert an entry into a table on master, so a timout occurs, the value of "Rpl_semi_sync_master_no_tx" added one.
3. Resume the network of slave, but the master never return to semisynchronous replication.
5. After executing "stop slave" & "start slave", then the master returns to semisynchronous replication.

Why the master can't return to semi-sync after the slave machine resume the network connection.
Any one help to explain it?