I encountered the same issue. After many attempts to insert different values into IL-1040 and the ICR Schedule, I did determine that they are working correctly. If you read the IL-1040 Instructions, you will see that there are a number of Exceptions that affect Tax Credits. This is why TurboTax inserts a 0 in Line 5 even though Lines 3 and 4g have real values.
From IL-1040 Instructions, Page 9:
Income Exceptions for the property tax and K-12 education expense credit
If your federal filing status is married filing jointly and your federal AGI is greater than $500,000, you are not entitled to a property tax credit or a K-12 education expense credit.
If your federal filing status is single, head of household, married filing separately, or widowed and your federal AGI is greater than $250,000, you are not entitled to a property tax credit or a K-12 education expense credit.
I hope this helps.
