Have you rearranged (or considered rearranging) your choir members' positions or do you just let them sit wherever they please? ... But, based on the problem you're having, I'll assume you don't have a choir full of singers, so perhaps it'll help to first identify the stronger singers and then assign them to buddy up with a weaker singer.
We have a praise team, about 6 members (2 sop, 2 alt, 2 ten). I have one strong alto and one strong tenor; so if someone is off, it's noticeable. Yes, I do pair them up and it works until one of my strong ones have to sing lead or is absent. One other thing I do is invert alot of their parts to make it easier for them.
What trips me out sometimes is that they'll request to sing alot of popular songs, and when we try one, I hear a bunch of "that's too high!, I can't reach that note!", etc...
So I try to stick to the easy stuff like "Lord You are good", "Let the glory of the Lord rise among us", etc...
But, I alway let them know that I appreciate their faithfulness. I'd rather have faithful ones who can almost sing, than good singers who aren't committed.