It might be a bug. I had one very similar to this when fighting against the Stormcloaks. Ulfric would have his last words before fighting him and then he would do nothing. His buddy Ragnar would do nothing at all. They just stood there, and there was no way to kill them. It sucked. I couldn't finish the civil war quest line on that play through.
ps. your description was not very descriptive. the more details there are the better it is for someone to help.