Each time you 'complete' a task you will notice that, when you open that task again, it will be marked as 'Not Started' and the attachments you have uploaded are not there. This is totally normal.
Think of a task like a book. Each time you complete the task, you have turned the page and are now on a new page ready to complete the task again. So, where are the previous pages, or completed tasks?
Open your task and click on the 'View Submissions' button (as highlighted in red in the screenshot below:
You will notice a 'Submissions' section below Attachments (as highlighted in red in the screenshot below).
In our example, you will see there are three submissions, each completed approximately one year apart. These are you previous completed tasks (submissions). To see the uploaded documentation associated with each submission, simply click on the relevant submission. In the example below, we have clicked on the bottom (oldest) submission and found the uploaded Gas Safety Certificate for January 2018 (#1 in the screenshot below). We can see that this particular submission was 'Last Completed' on 10/01/18 and is next 'Due' on 10/01/2019.
To look at other submissions on this task, simply click on 'View Submissions' again.