As writing has two, Task 1 and Task 2. In Task 1 of writing, try to find the reason why you are writing the letter. Break your questions and note the key points. It will help you elaborate on your letter. See if the letter is formal or informal. It will help you to maintain the tone in your letter. Lastly focus on linking words which will help you in the continuity of paragraph.
Task 2: Note the keywords and choose whether you agree or disagree/whether you are talking about the advantages or disadvantage and according to write the passage.

What exactly do you mean to say when you say failed. What is the score you are aiming, and what score did you achieve? 

There are a couple of things you can do to improve your band in the speaking module. 
Using idioms but relevantly can help improve your score. Similarly, using phrasal verbs and a useful vocabulary helps to fetch you a better band. 

Fluency is another factor. Speak as much English as possible with family friends and groups on social sites. Practice speaking on familiar topics like your hobbies, job, subjects, college, aim, home, family members and so on. Also, there is a wide range of cue cards available online and offline. Practice speaking on them. The more you practice, the more the ideas to express. 

Hiring a professional trainer to achieve your desired band is another option you can look into.

IELTS writing section can be difficult sometimes. You have to keep a couple of things in mind when it comes to writing IELTS essays or letters to get the desired result.

1. Concentrate on the structure of your answer. Your answers should be "structured". By structured, it means, there should be a flow of thought from one paragraph to another. The context should not change, and the answer should be highly focused on that particular topic.

2. You can use words like "however", "in addition to", "having said that", to conclude your statements. Also, to maintain the flow between the sentences and the paragraphs.

3. You can use unusual words. By unusual it means that the words that everyone knows the meaning about, but nobody uses usually. By improving your vocabulary, you can showcase your lexical proficiency

4. The most important part is to get your sentences grammatically correct, for example, you should know where to use had been, would have been etc.

5. You'll have to play safely while you're writing, ensure that you don't form grammatically incorrect sentences. Also, you do not deviate out of the topic.

6. Your punctuation marks should be on point. You should know where to put a comma, a full stop, and colons and semicolons.

Hope this helps!