That is a horrible macro to use, stick with <mo> only, ill tell you why.

If by chance you go out of range of the person you are trying to heal with that macro, it will cast it on yourself instead and you will not know it until it's to late, which could lead to a tank death. This is most likely what you are experiencing, being out of range of the person you are trying to <mo> heal.

If you are just using <mo> you will be unable to cast until you move close enough to be in range of the person you are healing, preventing you from accidently healing yourself instead of the tank and being none the wiser.

Bad bad macro, <mo> only.