I encountered an odd thing. I had ARD set to remote into a few workstations. I was doing some account cleanup on them and changed the admin accounts. In System Preferences Sharing, I still had it that all admins could remote in, but after deleting an admin account and adding a new one, I couldn't remote in with the new one.
There is a way to refresh it, however, which I found here. You ssh into the remote machine, and ten paste in these two commands:
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -activate -configure -access -on -restart -agent -privs -all
You should then be able to use ARD to remote in again.