Have you tried deleting and re-adding the client?
If you can not add the client back, what IP dose the client have? do you see another computer record in ARD with that IP?
if there is no other record with that IP, dose the user/pass your using on the client work on the client directly?
If you look at client in system preference sharing, is remote management enabled? is the accunt your using set to be allowed to remote connect? Including the options?
Is the firewall off or on on the clent?
in ARD try right clicking on them and changing the login and password
perhaps you have the wrong login wrong and password entered
I've also seen ARD "bounce" and refuse to add a machine
to fix have a look at the ip address of the machine you're trying to add
look in all computers and change the IP address of the existing computer
to something different (right click it)
then go back an add the machine that ARD refused