If you don't meet IRS substantiation requirements, your charitable deductions could be denied. To comply, generally you must obtain a contemporaneous written acknowledgment from the charity stating the amount of the donation, whether you received any goods or services in consideration for the donation, and the value of any such goods or services.
If you haven't yet received substantiation for all of your 2014 donations, you may still have time to obtain it: "Contemporaneous" means the earlier of 1) the date you file your tax return, or 2) the extended due date of your return. So as long as you haven't filed your 2014 return, you can contact the charity and request a written acknowledgement — you'll just need to wait to file your return until you receive it. (But don't miss your filing deadline; consider filing for an extension if needed.)
Be aware that certain types of donations are subject to additional substantiation requirements. To learn what requirements apply to your donations, please contact us.