What Causes Delayed Period After Taking I-pill Followed By Sex?

I had sex on 31 dec. 2014 and after about 17 hrs she took the ipill...and then again on 15 jan 2015 we had sex and she took an ipill within about 15 hrs.. she got her last period on 1 jan 2015 ...but not it has been 3 days and she is not getting her periods till now...please help!!!