1.9 Bug

I use the Minecraft.msi using java on windows 10. I have a skeleton, struck it once with my iron axe, then I charge in and it doesn't take damage. I assumed lag, I kept hacking and slashing at it and it doesn't budge, or do anything, just waving arms back and forth. I dug a 3x3 dirt hole under it, and it doesn't fall. Possible bug?