Not getting default option to close dialog box in recordactionitem in interface?

I think default there will not be any close option. You need to provide some button so that the related action process model flow can proceed to complete.

Add a button action to submit the interface or just add a cross icon using the richTextDisplayFIeld() and then add a submit link on it. 
Also, the default option to close will only be visible when you do not have a way to submit the form. So that means you must be having a button on the screen. 

Is the process model being launched using a Start Form or is it launching immediately to a chained User Input Task?  Appian doesn't publish the exact workings of the "close button" functionality seen (sometimes) on the dialog pop-up, but I suspect the difference might hinge on whether or not a Start Form is used.