I don't get it

Last night in choir we were learning the words for a Russian opera piece (Coronation Scene from Boris Godunov, by Moussorgsky). Overall, the pronunciation wasn't TOO bad, but it could have been far better. So, here's my question. Why does it seem to be so difficult to either string several consonants together (zdr) or to use the "soft mark", effectivelly adding a consonant "y" after a t, s, or r?