Sorry to hear that, I had the same problem. The way I eventually got around it was to imagine I was writing to an anally-retentive person who was trying to find an excuse to give me a poor mark. You should first make a vert direct answer to the question, and then stuff in as many additional details as you can think of.
Success One is an excellent book for refining this skill! Do the questions for each module, then check the answers. Note what you left out, rewrite the answer the next day without checking your notes to practise your composition skills.