Within my iOS apps, some parts (e.g., images, JavaScript, CSS, slider) of the website are not loaded (correctly).

Does your site use SSL, so the address starts with https instead of http? That would be recommendable. However, suppose the HTTPS page contains content (e.g., images, CSS sheets, or JavaScripts) retrieved via regular HTTP (instead of HTTPS). In that case, the connection is only partially encrypted: The unencrypted content is accessible to sniffers and can be altered by man-in-the-middle attacks, so the connection is no longer protected. A website that exhibits this behavior is called a mixed content page, to which WebView is very sensitive. Often the WebView does not load the insecure HTTP content, then the only thing that helps is to upgrade to HTTPS for all content. Please check the source code to see if you still load HTTP content.