Frontal branch injury typically temporary following Brow Lift

The dissection and movement of the tissues required to elevate the forehead and eyebrows during endocopic brow lift surgery can cause dysfunction of the frontal branch of the facial nerve, which innervates the frontalis muscle. This would lead to decreased function of this muscle.  

Fortunately, unless the nerve was actually severed during surgery (which is rare), its function should return over time, although it can actually take quite a while for some folks. For the most part, just hang in there, and you have a good chance of regaining some if not all function.