Are you sure the songs you're teaching are as easy as you think they are? I'm only asking because sometimes our perception can be off a little bit. Somewhere around here, there's a thread with songs easy enough for the musically-challenged.
Also, try giving them the songs ahead of time so they can get a feel for it. Even if they can't get the parts, they can still learn the melody and the rhythm, timing, lyrics, etc. That makes it a lot easier to teach.
And I also think you should try to do songs without leads, and when you do have to assign a lead, just try to do it fairly. I'm not really a fan of having one person (or 2 ppl) doing all the leads. I think it breeds animosity, and if you have problems already, the last thing you need is resentment and frustration or animosity in the choir stand.