Did American Missile Defense Fail in Saudi Arabia?

There is no such animal as 100% effective, so not surprised if something is missed. A miss does not failure make, for an entire system. A miss sucks if you're in the target area.
Think about it, how many times have you been firing a rifle or pistol and had a misfire, pop no kick or other ammo related incident where one of the simplest powder actuated firing systems invented fails to ignite the round and send hate down range.
Now add layers of complexity and electronics into the mix. The fact that the system can hit a comparatively small inbound weapons system moving at high velocity at all was until not to long ago virtually unachievable.
So while one gets through and it sucks to be on the receiving end think of the countless other examples where the damn thing was spot on and intercepted the incoming projectile.

Next they might but Russian...