Band 5 in Writing is achieved when a candidate partially addresses what is being asked , lacks clarity of thoughts, no proper conclusion drawn ,irrelevant ideas, inadequate paragraphing, bullets and numbering the paragraphs, repetitive sentences, lack of progression, overuse of cohesive devices, limited range of sentence structure and frequent punctuation and spelling mistakes in writing section. It is advised to work upon these areas and overcome these problems by regular practice .
My last advice for improving this section is stay patient as miracles cant be happen overnight. Apart from Writing on different topics, Keep on writing some lines on your own , Pick easy topics (leisure activities, hometown etc ) and move on to tougher ones i.e. Technology , education system etc step by step and you will see the change.