If you manually entered the cost basis for a holding (in a manual portfolio or imported portfolio), it will not automatically update the cost basis for the holding when additional shares are added.
You can manually edit your cost basis by following these steps:
1. Log in
2. Select the settings gear in the “Accounts” display on the left.
3. Select "Edit Holdings" next to the account which you wish to edit
4. Enter the correct "Cost per share" or "Total" Cost" for each holding.
5. Select "Save" and then "Close"