when i flexdown a running instance ,it does not work

there are 3 instances,when i call flexdown api,i found that it can not flexdown a running instance,there is still 3 running instanec,below is resourcemanager.log
uler.Rebalancer: Active 3, Pending 0
2015-02-09 01:11:21,831 INFO com.ebay.myriad.scheduler.MyriadOperations: About to flex down 1 instances
2015-02-09 01:11:21,831 INFO com.ebay.myriad.scheduler.MyriadOperations: Flexed down 0 of 1 instances including 0 staging instances, and 0 pending instances.