Hi Dongxiao,
There was a change in an upgrade of the X-Windows server on RHEL-6 that caused Ferret windows to stop responding. (A particular XWindows event is no longer being generated.) This bug was mentioned in the release notes for 6.95 and the freeze-up issue was fixed in the latest release. There is also a related issue with resized windows not updating which I cannot remember if it was fixed for the current release or if that fix is in this next release soon to go out. This behaviour is only seen when running and displaying ferret on your local system. So having a remote display does not show the problem (which is why I cannot test 6.96 at this time).
So the solution for the freeze-up is to update to the latest ferret, or to use pyferret which does not have these issues. It does surprise me that when your system was just upgraded from RHEL-5 to RHEL-6 that Ferret was not also updated for RHEL-6 (with the latest version). So do let us know if you are running the latest RHEL-6 version of Ferret and this problem.
Best regards,
Karl