There's a couple of things with that.

First, you should have a separate Interact button that does nothing but assist and interact. If you're trying to use the same assist/interact that you have tied to your DPS button, then you're also triggering the DPS commands, which will cause them to look for a target to attack if they don't already have a target.

Second, you might want to turn off Attack on Assist in your WoW options. Again, that attack command could cause your slaves to look for a hostile target to attack.

Finally, make sure they are actually assisting you before interacting so they are actually on your target.