Open transactions with a Forecast date before the Start date will not appear on your Cash Forecast Chart. This could happen if you edit the due date on a bill, but do not update the Forecast date.
To change the Forecast date on an open transaction:

Click the Home tab > Cash Forecast.
Click the link that says "Take care of open invoices, unpaid bills, and/or vendor credits for a more accurate cash flow".
Click Change for the Transaction you would like to update.
Select the Forecast date and click save.