You probably have corrupted transactions.

First try Rebuilding the file (File > Utilities > Rebuild Data). If you’re lucky, that’s all you’ll need.

If that doesn’t fix it, try searching for the amount you’re off. If there’s just one transaction causing the problem, this is usually the easiest way to find it.

If there are more than one causing the problem, you’ll need to do more research. Start by pulling a Balance Sheet report for All Dates and then select Month for Columns. Find the first month that it doesn’t balance, then change the dates to that month and select Day for Columns to find out what day doesn’t balance.

Then create a Custom Transaction Detail report for that day. If you Total the report by Account, it might be easier to identify the problem. Once you find the corrupted transactions, delete and re-enter them.

You can do this by entering the sales tax items into the sales receipt or invoice. Just be careful to clear out the tax at the bottom and use a subtotal item if you have more than one item per sales tax item because it only calculates on the item above it.