When an answer is made all regarding mails are not shown, so we need to know how to solve it.

When an agent receives an email from a customer, they should see the entire email thread.

If you mean from the customer end, most email clients show you the full thread as you'd expect. However, Microsoft works differently. The agent has the ability to include history within the email reply by clicking on the three dots towards the bottom of the screen. This will send the full thread history to the end customer, however it will show poorly in most email clients.