Thanks for submitting a question about responsiveness issues. There is no known issue with iPhone. BoldGrid is tested on numerous devices, but customization of a site means different page elements and media may require tweaks to align just right. Every user and device is different.
Also, browsers are programmed differently across platforms. Safari, unique to the iPhone, may render a page slightly differently than on Chrome or Firefox. Likewise, these same browsers on Android will likely also render pages differently.