Struggling Voice Samples

I want to write some simple choral music and I see that very often the tenor part plays easily and comfortably at least an octave above middle C.  I've bought some high quality Solo Voice samples from VSL. The problem is that when they go anywhere near that sort of range the tenor sounds strained and struggling and it doesn't sound very natural. Other voices also seem to struggle in their upper registers.  Is there a reason for this?  Is it a known issue with these samples that this problem arises? What can I do about it?