Hustle Plugin - Complaints from clients

I've had several complaints from clients who have the Hustle plugin installed. They say their mobile visitors cannot close the pop-up window. I've tested this myself and I see that it takes several attempt to click the X close button. Seems that this X is unresponsive to touch or slow to respond to touch gestures. This issue is not isolated, I've tested it on other sites using the Hustle plugin and they are all the same, slow to respond to touch gestures on mobile. My testing was done using iPhone and iPad, both devices had the same issue.