After making the changes in the host name you will need to restart the required services showing in the ambari UI as "restart required"
However while specifying hostname (FQDN) there you will need to make sure that the mmentioned hostname should be resolved by all the cluster nodes and from Ambari server. Also better to check and verify the output of the following command on the resource manager host to see if we are getting desired hostname at the host level or not?
# hostname -f
Also if you can share the exact error that you are getting while opening the Capacity Scheduler view then that will give us more idea about the issue.