There are differences between a Band 6 task response and a Band 7 response in IELTS writing section. Do you know what these differences are? Unless you know, it would be just impossible for you to upgrade your band score from Band 6 to Band 7.
Let us try to understand these differences with a question:
Question:

You were travelling via domestic flight to a place outside your home town. But when you reached your place of stay, you found that you forgot your travel bag in the airport. Write a letter to the airport authority regarding this and in your letter,

• Say why you are writing
• How you lost your bag
• What action you are requesting from authority

Following is the response of a student who got 6 Band in writing:
Band 6 Answer:
Dear Sir,

I am writing to inform you about my suitcase which I lost in your flight during my travel.

I am frequent traveller of your airlines. Last Sunday, I took flight number 464 from Banglore  to delhi at 10 o’clock in the night I was carring one laptop bag and one suitcase and I put the same in the cabin only. The flight reached late in delhi around 1 o’clock in night. I was very tired due to hectic day schedule. I took my laptop bag and forget to take my suit case before leaving I already this to your claim officiers.

It is black in colour and brand name is VIP printed on Left. It is four wheel trolly bag new model launched by company. It contains my important official documents, such as two blue colored fils, two green spiral binded manuals, one planner from hilt, one imported pen from parker and other few documents. In addition to that there are few clothes as well.

Please find it on urgent basis and do inform me on my number once you are able to locate it. Please send it to my below mentioned home address

Sushal Lok,

Phase-1, Gurgaon
I look forward to hearing from you.
Yours faithfully
Rajesh Verma

(211 words)
Now, let us change this answer to Band 7 task response.
Let’s first change the first body paragraph:
You may make following changes to make it better:

• Removing grammar, spelling, capitalization and punctuation errors such as Banglore, carring, “around 1 o’clock”, “and forget to take my suit case” etc.
• Replacing with better words e.g. “kept” in place of “put” and defining exact location “in the cabin under my seat only” in place of “in the cabin only”.
• Removing something extra e.g. “I am frequent traveller of your airlines”
• Adding vocabulary e.g. unexpectedly, heavy-eyed etc.
• Connecting the idea by changing the order of phrase e.g. “Before leaving, I took my laptop bag though forgot to take my suit case.”

Let’s change the second body paragraph:
You may make following changes to make it better:

• Mentioning the lost item at the start of paragraph e.g. “My suitcase” in place of “it is” and specifying location exactly e.g. “on the left top corner” in place of “on Left”
• Removing something extra e.g.“It is four wheel trolly bag new model launched by company.”
• Shortening sentences e.g. “It is actually a trolley bag containing my important official documents, such as two blue and two green files along with one imported pen from “Parker”.”

Let’s change the last paragraph:
You may make following changes to make it better:

• Adding a phrase to make it better e.g. “I request you to”
• Changing phrases for making it better e.g. “kindly inform me once found so that I could collect from you.”
• Finally, removing complete address as it exceeds word limit

Following is how the final Band 7 answer would look like:
Band 7 Answer:
Dear Sir/Mam,

I am writing to inform you about my suitcase which I lost in your flight during my travel.

Last Sunday, I took flight number 464 from Bangalore to Delhi at 10 o’clock in the night. I was carrying one laptop bag and one suitcase which I kept in the cabin under my seat only. Unexpectedly, the flight reached late in Delhi at around 1 o’clock in the night and I was very tired and heavy-eyed due to my hectic day schedule. Before leaving, I took my laptop bag though forgot to take my suit case.

My suitcase is black in colour with brand name “VIP” printed on the left top corner. It is actually a trolley bag containing my important official documents, such as two blue and two green files along with one imported pen from “Parker”. In addition to that, there were few clothes as well.

I request you to please find my suitcase as soon as possible and kindly inform me once found so that I could collect from you.

I look forward to hearing from you soon.
Yours faithfully,
Rajesh Verma

(183 words)