Why do some quests not work properly in World of Warcraft?

Basically im a night elf and i tried to complete a quest where i had to meet dentaria silverglade and i went to the marked location and she wasnt there! All i saw was a striped nightsaber and also i tried another quest where i had to use arcane missilees on two training dummies, i did it on 8 and still it says 0. This is getting really annoying, please help.