The device is going into current limit on the input side. For those specifications this part can only deal with approximately 650mA load current.
Our online simulation tool EE-Sim is helpful for further analysis: http://maxim.transim.com/pwm2/designreq/Index?partId=9689
This device has a selectable inductor peak current limit of 1.85, 2.7 or 3.5A. Make sure to stay between these limits when completing your design.