Yes, you need to remove that focus from the DPS macro. As I stated in my first response, it's not working because your focus isn't your tank, but your DPS key keeps telling you to assist your focus {the mob you CC'd} and attack. Even if ISBoxer is still sending a command to assist your tank, the macro itself that you are using in WoW is making you reassist that focused target.

What I guess is happening is you focus your target and CC it, then when you go on to DPS non-cc'd targets, ISBoxer is sending the default command to assist your tank. Then it triggers your macro, which is sending both the command to cast whatever step in your castsequence AND the command to assist your focus target {the CC'd mob}. But because your focus has no target and thus nowhere to assist while it is CCd, the ISBoxer assist is "winning", just like if you were to manually target something, start casting a spell and then quickly switch to another target or clear your target before the spell completes casting. That spell will still land on the old target.

Once you hit that focus target as the DPS target, you're getting into a sort of "assist loop" that won't allow the sequence to go off because your tank has a target and ISBoxer is telling your slave to assist the tank to aquire that target, but your focus target is awake now and also has a target...your tank, which is a target you can't cast harmful spells on. Now your WoW macro is "winning" over your ISBoxer assist and your DPS stops dead.

Because of CC requirements, I moved away from using my tank as focus so I could clear up setting a focus for CC. Basically, I pick a target on my tank, mouse over whatever party member should CC the target and hit F4, and that party member marks and focuses the target. I do the same thing for however many mobs I need to CC. Once they are all set up, I hit F4 again while not over the party frames and they all CC at once.

So instead of focus, all my macros use either /assist party1 or /castsequence [@party1target]. But since you are using ISBoxer and it has assist built in, you shouldn't need any assist function built into your WoW macros. Just use the one ISBoxer has and you won't have these conflict issues.