This is more of a balance issue than a bug, I'm thinking, Iron golems are too slow to approach the skeletons and/or the skeletons shoot too often. Knockback slows the golems down even more and they never get to the skeletons. Even just one skeleton is enough to overpower one golem.
Environment
Windows 10, Java 1.8.0_60, Singleplayer Creative
Possibly working as intended since the skeleton AI has been updated to move backwards/strafe.