Recently, I have bought a choir sample library from a major manufacturer. The samples themselves are top-notch, the dynamic crossfading excellent. However, I'm a bit disappointed by the quality of the (monophonic) legato that I've been able to get out of the library. The transitions between notes that are supposed to be played back legato often sound harsh and unrealistic, with a kind of upward "swoopy" artifacts.
So my question is: how to get good legato from a choir sample library that has special legato samples? I've already tried a number of things, such as slightly overlapping the notes in my DAW, or changing the legato speed and the "attack" of the notes. The strange thing is that some legatos sound really good (especially if the notes are a semitone apart), while others sound awful and fake (on leaps greater than a minor third, for example). Also, the legato seems significantly worse in ascending passages. Why would that be the case?
Any insights would be much appreciated.