Im looking at the instructions and see exterior windows get 30% but are limited to $600.  Is it not giving you the $600 credit on line 20b?

This is from irs.gov

The maximum credit you can claim each year is: $1,200 for energy property costs and certain energy efficient home improvements, with limits on doors ($250 per door and $500 total), windows ($600) and home energy audits ($150) $2,000 per year for qualified heat pumps, biomass stoves or biomass boilers.