Hi Dave, There are a few things that could cause the issue, but you will need to call phone support since it requires an engineer.
We have an FS7600 where the battery was going bad on one of the controllers.  So I detached it and it went offline.  We pulled the controller and replaced the battery.  We plugged it back in and when it was ready we tried to "attach" the controller and it has been sitting at 23% for hours.  Not sure what to do.  ANy help would be appreciated.
FS7600 Stuck trying to Attach Controller