Why is my energy credit not allowed?

The Energy Efficient Home Improvement Credit (which includes insulation) is not refundable, so if you have no tax liability, the credit is not used. 

Certain tax credits are applied to your tax liability before the Energy Efficient Home Improvement Credit.

The credit is also not carried forward. 

Please see Instructions for Line 31 on Form 5695

If you have not added all your income yet, the credit will be recalculated as you add income and make other changes to your tax return, so that if you owe tax when everything is entered, you will get the credit.

But if you have entered everything, and owe no tax because you got a lot of other credits or have low income, then you won't get the energy efficiency credit.