It's a huge subject. Problems could accor because of the phonology of the listener's language, where they have trouble hearing sounds because they don't exist in their language,like the difficulty the Spanish often have with hearing 'ship' and 'sheep'. The phonology of spoken English, with intrusive letters, missing letter, schwa sounds, could also cause problems:
http://www.usingenglish.com/cgi-bin/...d&bool=and