Hello,
There are a few issues that may cause this problem. We will need some more information to help you find the cause. Two things I recommend you check (and post if you can):
1) Check the /etc/messages log on your controllers for any errors related to your SQL Servers. Post them here if you can.
2) Check the Windows Event logs on your SQL Server host servers.
Between those two information sources, hopefully myself or someone else reading can help you find the cause of your issue.