Reassessment for failure to report investment income

[lg-w]

I received a notice of reassessment by the CRA, for failure to report investment income on T3 and T5 slips. During data entry I gave permission to Turbotax to retrieve that information from my investment institution. I assumed that the information was complete, but now I am learning that I did not report some of that income, resulting in a large fine by CRA.
Why was the data that was retrieved incomplete and how do I find out which portion of the income was incomplete?

[TurboTaxJennifer]

The information imported would have been the information available at the time of import as provided by your investment institution. To find out exactly which information was not included, compare the slips currently listed in your CRA My Account to the entries on your return. Select the option for tax slips from the "Tax Returns" tab of your CRA My Account.

If you are not a CRA My Account holder, use figures from your paper slips or contact your investment institution for a full list of amounts.