This might happen because you were not aware (or forgot about) various transactions, or that transactions you predicted did not occur on the date and/or in the amount predicted. To rectify this you have 2 options:
To amend/add/delete the relevant transactions through the Money In/Money out pages
To enter a new opening balance date and amount on the Opening Balance page. Please note this is a “nuclear option” as it will delete all the transactions before the new opening balance date.