This is often the case to why you're having trouble. A lot of people with pets/charmies, such as special guards or necro zombies, can order those pets to rescue them.

Now, if they have autoassist on, they'll still be in combat and you can 'hit target' immediately after they are rescued out. But if they have autoassit off, they will then be out of combat and you'll have to use some other type of skill/spell to reengage them to then 'hit target' - 'hit target' does not work as a way to initiate. And you can't murder or kill them either because you're already engaged with something else. E.g., dirt kick, a spec skill, a magic spell, a commune, to get them reengaged with you.