Basically a working report is a summary either of what you have observed needed doing by others (or yourself) during the week and what you have done towards this, or simply what you have achieved that particular week and the action plan you have in mind for the following week.
You do not say what kind of work you are into, so it makes it difficult to advise you more specifically.
If you are on a project for instance, you state how much has been achieved, what difficulties or problems have arisen (if any) and what is planned for the following week.
An engineer would say for instance:"This week the plan was to carry out task A and task B . A third of this was achieved but difficulties XX have arisen due to the following reasons. Therefore the action plan for next week is to carry out such and such to solve the problem then to proceed with the following tasks, etc...."
Use words like "aims", "objectives", "obstacles", "lack of information", "physical difficulties", "unexpected delays", "achievements", "successes", "time frame", depending on your own context .
I hope this helps you a little.