Do you think you could provide some samples of them singing so that it would be easier to determine the cause?
The only advice I can give right now is to maybe look at the crossfades (the buttons at the top such as p2p3 and acpt tend to help) or try a different resampler? You might even have to change to oto. Still, it's hard to recommend putting in a bunch of extra work when that might not be the problem, so a sample would really help.