DBinAL

A couple of months ago I had to replace my headlights. I centered the up and down aim by adjusting Screw A until the bubble was in the middle. Recently I noticed that both of the headlights were aiming down. On both headlights, the bubble was toward the engine - almost out of sight. I tried to get the bubble to move by turning Screw A - both directions but nothing changed.

What may have caused both headlights to get out of whack and how do I get the bubbles back where they belong?