I run into this all the time! If I had a nickle for every time I have said, 'You have to say all of the word!', then I... Well, anyway...
I think/guess it has to do with a difference between Chinese and English. Chinese words typically do not end with consonant sounds, so students tend to leave off consonant endings. I suggest you teach your higher-grade students about word linking- how final consonants are joined with initial vowels in following words to make them more clear. I think also Chinese words do not have prefixes or suffixes- extra words are added if needed to make the modification. Best strategy I can think of is to hammer home how important these details are to success on this oral part of the exam. Some will 'get it' and work hard and succeed; others will not and blame you for their failure. "Twas ever thus."
It seems teachers have the same problem all 'round the world: We find ourselves focusing more and more on helping students pass an exam and less and less on actually teaching something useful. Hang in there!

English Teacher