You're doing something wrong, sorry to say. Switch targets and click Obey again, your summon always switches immediately to that target. I do this all the time in dungeons to focus on a certain mob, or on an add if the boss is invulnerable or there are more important things to burn down. The only Macros I have on SMN is a Swiftcast-Rez with announcement in party chat, and the same thing for E4E and Virus, minus the Swiftcast obviously. Only to prevent pointless doubling up though. I find Macros to be infinitely more of a hassle, or even getting in my way and hurting my overall DPS, rather than helpful. There would probably be a higher amount of better players if some didn't seem quite so dependent on macro-ing so much.

Hope for the best, don't see many main SMN's out there so it's nice to see them around. =)