Pneumatic Drill malfunction
From Worms Knowledge Base
This glitch can occur when a worm stands on top of another and uses the drill on top of him, but it only occurs in particular circumstances. The result is that the worm below is not hit, and the worm doing the drilling does not move.
To quote Deadcode, "It happens when the bottom worm has fallen onto the land (and therefore the fractional part of its y-coordinate is 0.999985), but the top worm has either not left the ground since initial autoplacement, or teleported directly onto the land (so its fractional y-coordinate is 0.000000)."
When this is the case, the drill does not reach the bottom worm, but the top worm is still treated as standing on something, and so does not move. This glitch is planned to be fixed in the next beta update by extending the reach of the drill by 1 pixel.
Update: The glitch was fixed in v188.8.131.52, by forcing the worm using the Drill to fall down (slowly) to y-coordinate 0.999985. As it turns out, that fix does not work in all circumstances where the bug can occur. A worm that flies through the air with the Drill activated may graze over a worm, touching it (and making a drilling sound) without doing any damage.