Can you help me with retained earnings balance is reflecting for 3 months in 2019-20 balance sheet. It is not reflecting for entire year. Due which BS mismatch is arising

Glad to have you here in the Community, @business unit.

You have to run reports to see data of your retained earnings. Let me show you how

If you'll notice a balance for retained earnings in the Balance Sheet report, it is the Net Income from the previous year. The system closes income and expenses to start the first date of next year to zero. 

 Let's compare these two financial reports. First, run your Profit and Loss report for this year and see the Net income. 

1. Go to the Reports menu.
2. Scroll down under Business Overview and click Profit and Loss.
3. Look for the Net Income below.

Then, run the Balance Sheet report with the same year, you'll see the same Net Income under Equity. Refer to the steps below:

1. Navigate to the Reports menu.
2. Go to the Business Overview section and select the Balance Sheet report.
3. See the Net Income under Equity.

For further info, let's change the date in the Balance Sheet to the first day of the new Fiscal year it should be one date later than the Profit and Loss. Once you run the report, you'll no longer see the Net Income line (except if there's an activity in the P&L for that date) and you'll see Retained Earnings is keeping that amount. 

If you still get the same result I suggest consulting your accountant for further accuracy of your books, and any accounting helps.

For more details about this account, you can check this article: How to view Retained Earnings account details.

I also added this article for more tips about handling your financial reports, you can open this link: Run reports in QuickBooks Online.

If there's anything else I can help you with about retained earnings in QuickBooks. , you're always welcome to get back to me. It's always my pleasure to keep you up in running.