The most likely explanation is that you have not yet entered enough other itemized deductions to make your total itemized deductions more than your standard deduction. An additional donation will not make any difference in your tax or refund until your total itemized deductions are more than your standard deduction.
Or you might not have enough itemized deductions to exceed your standard deduction. The standard deduction is pretty high now. A lot of people who used to itemize no longer do.
Is Schedule A in the list of active forms in your return? If not, you are using the standard deduction.
