The hitbox on skeletons is bugged.

If skeletons are in a 1x space (as in a monster farm) you can only kill one skeleton at a time.

You can hit one skeleton, then you have to wait until the falling-and-dying animation finishes before you can hit another skeleton.  You should be able to hit skeletons continuously.

Example: swinging with the sword repeatedly will only hit one skeleton and will not hit again until his death animation is complete.

Thank you!