Aggro is a tricky system that I haven't fully figured out yet. Here are some of my speculations though.
The first determining factor of aggro appears to be defense. Not specifically Rec, but any additional defense you gain through lapis, buffs, etc, count towards your aggro. I think that this is the factor that trumps all because it is one of the few that you actually have any control over.
Another contributing factor seems to be damage dealt. When I'm with a Mage on my Defender, I usually have no trouble unless they have Rec, but I was once struggling to find a party, so a guild member, a 58 UM Fighter, came to fill in for my DPS. I had trouble holding aggro against her, even though she had very little defense. My speculation is because of the huge amounts of damage that she dealt.
From what I've seen, some classes are just naturally aggro magnets. More than others, Mages/Pagans seem to have a higher aggro value for their defense. It doesn't take much defense for a Mage or Pagan to steal mobs off you, but even for a pure Rec tank, getting them back may be difficult. This may be due to my next speculation though.
Types of attacks
If this speculation is correct, than different types of attacks generate different levels of aggro, separate from other factors such as defense. From what I've seen, magic attacks seem to generate the most aggro, followed by shooting, and than finally melee attacks appear to hold the lowest aggro value. This speculation has not been properly tested yet though, as least not by myself and I don't think I've read anything about this.
Frequency of attacks
How often you attack seems to gradually build up to acquiring aggro. I was duoing with a guild member, my HM Mage and her UM Priest. Both of us have Rec in our builds. We had roughly the same attack power, but she had more defense than me. Still, aggro switched hands often because she played the tank role and attacked less often than I did.
These factors are what I have found contribute to aggro. The problem you're having may have something to do with one of these, if it's not a problem of your technique or your defense.
Like I said about defense though, it is one of the few things that you have control over so it is usually your trump card. Once you get your AoE debuffs, you can use those to increase your frequency of attacks, but for now, all you can really do is increase your defense.