When a responsive mobile design does not work on iPhones but works elsewhere, the issue may stem from several factors specific to how iOS interprets Web Design:
Viewport Issues: Ensure the tag is correctly configured in your HTML. This tag helps iOS understand how to scale and display content.
CSS Breakpoints: Verify that your CSS uses breakpoints compatible with iPhone screen sizes. Sometimes, media queries might not cater to iOS screens due to variations in device width or pixel density.
Device-Specific CSS: CSS rules specific to iOS or specific device width/heights may conflict with other styles. Make sure there are no device-specific hacks that could be disrupting the style.
Rendering Engine Differences: iOS uses WebKit as its rendering engine. Elements may behave differently compared to other browsers. Test designs using Safari, which shares WebKit, and refine CSS/JavaScript to handle such discrepancies.
JavaScript Issues: Scripts can sometimes behave unpredictably across different platforms. Using feature detection rather than browser detection and ensuring scripts are cross-browser compatible can help solve function inconsistencies.
Safari-Specific Styles: Safari may apply default styles to certain elements affecting responsive designs. Reset or normalize CSS files can resolve conflicts introduced by default styles.
Testing and Debugging Tools: Use built-in developer tools on Safari or an iOS device to inspect elements, console logs, and network requests for iOS-specific errors or issues.
Clipping and Overflow: A common issue where elements aren’t properly resized or overflow out of visible bounds. Check CSS for overflow and flex related attributes to ensure content scales properly on viewport changes.

By addressing these potential problem areas, it is likely to remedy the issues causing the responsive design to fail specifically on iPhones. Testing across multiple iOS devices and versions is also recommended to catch any inconsistencies.