If the Opening Bank Balance reports as Nil:
Check to see if the Nil balance is being reported correctly from the closing balance of the previous period by running a Balance sheet report
If it is Nil in Calxa, check if it the Balance sheet is correct with the data in the Accounting file
If the Accounting File is different to Calxa, run the Sync to bring the data through to Calxa
If the closing balances for the previous period in the accounting file are also Nil, this may need to be updated by adding a Journal to create opening balances.