I've been having this same issue with the Mortgage Interest worksheet and think I have it figured out.  When entering the amount for "Interest paid in 2020" line, you'll notice that the "Months loan outstanding" automatically is entering "12" months".  The months loan outstanding is not allowing you to change it unless you override it.  If you have more than one loan it's important to have the correct amount of months each loan was outstanding to allow the "Average balance" for the year to calculate correctly.  What is happening all loans entering are calculating at 12 months which is causing the Average Balance to increase thereby disallowing some interest.  This is not correct.  I tried overriding the Months loan outstanding to the correct amount of months for each loan and unfortunately the program is always calculating as if all loans are outstanding for 12 months.  I have not been able to find a workaround other than not using the worksheet..