There are a couple of things that can cause the Alternative Fuel Vehicle Refueling Property Credit to be reduced to zero.
First, the credit is non-refundable and you must have a tax liability in order to take advantage of the credit. Also, the calculation of possible Alternative Minimum Tax (AMT) is involved in the calculation of the credit and may reduce it to zero.
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.