Hi Navin - Welcome to the Atlassian Community!
Previously Done issues might have been released. If so, they would disappear from the board.
Having said that, check the sub-filter that is used for the board. If further narrows down the Done column (like when a release is done).
Go to Board Settings > General
The Sub-filter will be near the bottom of the screen.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.