Hi keating.jan, 

Thanks for posting in the Community!

We can compare the amounts of the previous BAS lodgement to the BAS period you're trying to lodge that way we can identify the line code on your BAS that's causing the difference. First thing we're going to do is to check the unlodged BAS Summary. Any unlodged BAS periods can be found in the GST> select History tab (see BAS marked as Unlodged)> then click on View BAS summary.

Then go back to the GST tab> click on Run reports dropdown arrow and select GST Summary or PAYG Withholding Summary. You can compare the amounts of that report to the unlodged BAS Summary, make sure that both reports have the same report period.

Lastly, you may want to compare as well the PAYG Withholding Summary and the Detailed Activity report and ensure that they are matched.

I would also suggest to please contact our Customer Success Support Team to help you further check on your BAS/GST reports in QuickBooks Online.

Feel free to post again if you have any other questions and we're here to help you.