My apologies -- I misread the question.
The most likely reason that you are not being allowed to claim the credit pertains to Alternative Minimum Tax (AMT). AMT is a part of the calculation for the credit claimed on Form 8911, whether your particular tax return is subject to AMT or not.
There is a tentative minimum tax for AMT that is calculated in the background as your return is completed. Form 8911 compares that tentative minimum tax to the net regular tax after certain other credits have been taken into account. If the tentative minimum tax is greater than the net regular tax, then the Form 8911 credit is disallowed.
Unfortunately, the instructions for Form 8911 do not specifically spell this out so that it can be better understood or explained.
For your reference, here is a link to the instructions for Form 8911: Instructions for Form 8911
@michaelreeve
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