Facebook scrabble accepts word in dictionary but not when i arrange the letters?

kay so i've been playing scrabble with some random people from facebook for a while. sometimes id type in a word into the dictionary, and it comes up as valid, but when i go to arrange it, it says 'invalid word.' what is the right way to arrange the letters? and what are those arrows for when you click a blank tile and the direction changes? some words that i couldn't arrange were taken by my opponent, and she was able to touch multiple tiles in different directions. what am i doing wrong?