Failed with error 'could not acquire collection lock.....

各位大大好，麻烦帮忙看个问题：
balancer:
Currently enabled: yes
Currently running: no
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
8 : Success
1 : Failed with error ‘could not acquire collection lock for mongodbtest.usertable to migrate chunk [{ : MinKey },{ : MaxKey }) :: caused by :: Lock for migrating chunk [{ : MinKey }, { : MaxKey }) in mongodbtest.usertable is taken.’, from shard0002 to shard0000
databases:

麻烦帮忙看下这个Failed是什么意思？为什么会有？

先谢谢了。