http://everquest.allakhazam.com/db/spell.html?spell=728
Description
1: Frenzy Radius (5/60)
2: Reaction Radius (10/60)
3: Pacify
Obviously this is spell data from Live and not P99, so if someone wants to parse the DAT file to get the actual numbers, but for example... the Frenzy radius is 5, which means if you get within 5 units of a mob lulled with this spell, it will aggro you. The Reaction radius is 10, which means if you pull a mob (lulled or unlulled) within 10 units of a mob lulled with this spell, the lulled mob will still aggro you. The 60 means it works on mobs up to level 60 (which won't be the case in Classic, so this should be the only part that's not correct).
As a comparison, the ranger's Harmony spell (on Live anyway) has a frenzy/reaction radius of 15. I know that the two mobs outside the castle guarding the bridge in Mistmoore are within 15 units of each other, because they will both come despite being harmonied.
Try your song on a camp that has some mobs spaced out a little more and see if it works. Or post a screenshot of the mobs you are trying to Lull and we can probably judge if they're too close or not to be outside a reaction radius of 10.