JIRA does not fire any issue event when some of the fields are updated. Following fields won't get issue update event:

Changing issue link does not fire an issue event in Jira lower than 7.0.5

JIRA server does not fire an issue event on an issue link update in versions lower than 7.0.5 version.

You should update any other field on the issue to fire an issue event. It will start the synchronization.