Melee Attack isnt working?

[Leezord]

When I target a mob...and hit the Melee attack button nothing happens. I have to click another target and then re-target the original mob for melee attack to work. When mob gets low on health and runs off, I have to target a different mob and then re-target the running mob for melee button to work.

Any suggestions on how to fix this?

Same thing happens if I join an group and make an assist key with /attack on?