You guys experiencing Stern drops bricking, I experienced it too on my ACDC Luci and my Walking Dead (Iron Maiden, oddly.. not so much), and I made it a lot better on the ACDC and Walking Dead by doing two things you're free to try:
1.) I unhooked the bottom of the spring that pulls the drop targets down, and I pulled the spring a little tighter, then I hooked a portion of the spring about 3/4 of the way down the spring, increasing spring pressure. This helped pull down the drop targets a little more forcefully.
2.) I went under the playfield, and loosened the drop target assembly bracket, then I put shims under the rear of the bracket only (the opposite side from which the balls hit the drops). The shims were about .020" thick or so. Why did I do this? If you look at the face of the Stern drops, it's angled to prevent air balls (thicker at the top). In theory, this is a good idea. However, I think that when a fast ball squarely hits the drop target, the angled face of the drop rides up surface of the ball as the ball and the drop are both heading towards the rear of the slot in the playfield that the drop target is extending through. The force of the ball will force the drop and the ball against the rear of the slot, or against the washer and screw retaining the drop target, all the while the angled face of the target is supported by the ball, which is preventing the drop target from falling. The target and ball then rebounds at a rapid pace while still pressed together, and the drop target just magically sets right back down on the support shelf, leaving you to wonder what the heck happened. Tilting the entire drop target assembly back away from the ball strike direction, lessens the drop target's angled face, which helps it drop when the ball is pressed hard against it. You'll probably still have an occasional brick, but it won't be as bad, at least in my experience. Double check the leveling of the top of the drops to the playfield surface and level as required when you're done with this.
Your mileage may vary, but hey, give it a shot.
Jeff R.