If your DPS works fine, up until the point where a specific toon makes a mob their focus.
And it continues to work for other toons, but stops working for that specific toon.

I would guess that your assist is focus based.
On your DPS key.

You could check if Focus is getting in the way with a simple macro on the messed up toons.
Something like /clear focus (or it might be /clearfocus).




If you're using IS Boxer, check your wow DPS macro to ensure it does not have any kind of an assist in it.
By virtue of IS Boxer's addon, FTL assist is done for you.
If you drag a spell or macro (no assist in it) onto "1" for each of your toons, and then push 1.
They each do the spell or macro and auto target whatever the current window is targeting.
(This is only for keys in the FTL DPS Keymap, or for keys that have been added and include the Keymap Action > FTL Assist Me).

If your macro specifies a target, such as your focus...
IS Boxer will receive the "1".
Have all of the other windows, assist via FTL, so they're all targeting whatever the lead is targeting.
Then send "1" to every window.
Which would run the macro for the toons, that is on that keybind.
With the macro telling the character to cast at their focus, or possibly assist their focus and cast, or whatever it says to do.
If the macro says to assist their focus, and their focus is a mob...

Assuming this is the case, and you remove the assist from the DPS macro, it should work fine.
You push "1".
Everyone assists the active window.
There is no assist in the macro to change target (unless you want there to be).
And your DPS goes off fine.

Again assuming this is the case...
You might want to make a couple of mapped keys, for each CC toon.
The first is toon specific, it has the toon assist the active character and set that target as their focus.
It might be a good idea to raid icon mark the target (which will work on hostile mobs, but not on hostile players; even if the marking fails the focus won't).
Then you have a second keybind, which can be common to each CC toon....
This one will be to the effect: /cast [@focus,exists] CC Spell.
In this case, directing the attack (CC ability) at a target, via macro, instead of the FTL DPS target is desired behavior.