Why is the previous years balance shown as nil even though there was a balance in the opening of the current year?
If previous year balance of a party shown as nil but in current year opening showing the balance, then might be changes made in last year after creating the current financial year.
To resolve this issue you need to run Update Master Balances from the housekeeping menu in last year then change financial year again from the Administration menu.
Note: Take backup of your data before running this process.