A jumpscare is ineffective if the player expects it. A jumpscare is cheap if the player had no way to predict it. Both are bad, but ineffective is worse. A good jumpscare is one that catches the player off guard, but the player blames herself for lowering her guard.
The best way to make jumpscares effective is to not overuse them. Take your old film projector example. Played out like you describe it, everybody is going to expect a jumpscare. So let the creatures wander off again. Repeat a few times. Distract the player with something else. Only then, when the player has lowered her guard, throw the jumpscare at her.