Problem/Bug Scoreboard Objective teamkill.color Doesn't Increment

I had some commands set up to test for a player getting a point for killing a member of blue team -

/scoreboard objectives add juggkill teamkill.blue

/execute @a[score_juggkill_min=1] ~ ~ ~ etc

Tested the whole sequence several times, trying to isolate the part that didn't work. I eventually found that the objective wasn't incrementing when the player on Blue team was killed.
After much frustration, I switched the server over to Vanilla and everything worked perfectly.
Does anybody else have this issue?

Plugins installed:
mcMMO
PermissionsBukkit
EasyRules