Slime /entitydata motion causing unkillable slime.

I have been messing around with slimes, and when using this command on a fill clock:

/entitydata @e[type=Slime] {Motion:[0.0,0.0,0.0]}

They will float in the air, which is what I was going for, but the thing that is weird is that when you hit them with a sword, if the damage doesn't exceed their full health, they cannot be killed and just seem to not lose health when the damage is not above or not matching their health.