U.S Customs and Border Protection have advised that changes in ESTA application processing, results in real time approvals no longer being available.
International travellers using the Electronic System for authorisation for travel to the United States should apply as soon as possible but not less than 72 hours before their international flight is scheduled to depart. Failing to do so may result in denied boarding.
Those not eligible to use the visa wavier program will need to process a visa before travel.
For more information https://esta.cbp.dhs.gov/esta