title: WordPress's wp_get_attachment_image_srcset is not returning all the image srcset sizes
body: <p>I have a small function in my WordPress theme that returns responsive images - or at least, it is supposed to. I wanted to add some additional sizes to the images, so I used "add_image_size" and regenerated the thumbnails. But I can't get the larger sizes to show up in the coding. </p>
  
  <pre><code>add_image_size( 'iphone', 450 );
  add_image_size( 'phone-large', 650 );
  add_image_size( 'tablet', 850 );
  add_image_size( 'laptop', 1400 );
  add_image_size( 'laptopretina', 1600 );
  
  function images_fox($imageID){
      $img_src = wp_get_attachment_image_url( $imageID, 'full' );
      $img_srcset = wp_get_attachment_image_srcset( $imageID, 'full');
      $alt = get_post_meta( $imageID, '_wp_attachment_image_alt', true)
  
      return '&lt;img src="' . esc_url( $img_src ) . '" srcset="' . esc_attr($img_srcset ) . '" sizes="(max-width: 300px) 450px, (max-width: 600px) 650px, (max-width: 800px) 850px, (max-width: 1024px) 1024px, (max-width:1350px) 1400px, (max-width:1550px) 1600px, 2300px" alt="' . $alt . '"&gt;';
  }
  </code></pre>
  
  <p><br/><br/><br/>
  <strong>The laptop and laptopretina should both be showing up in the srcset, but instead I get this the following:</strong></p>
  
  <pre><code>&lt;img src="/sp-content/uploads/2018/08/banner.jpg" 
  srcset="/sp-content/uploads/2018/08/banner.jpg 1920w, 
  /sp-content/uploads/2018/08/banner-300x150.jpg 300w, 
  /sp-content/uploads/2018/08/banner-768x384.jpg 768w, 
  /sp-content/uploads/2018/08/banner-1024x512.jpg 1024w, 
  /sp-content/uploads/2018/08/banner-450x225.jpg 450w, 
  /sp-content/uploads/2018/08/banner-650x325.jpg 650w, 
  /sp-content/uploads/2018/08/banner-850x425.jpg 850w" 
  sizes="(max-width: 300px) 450px, (max-width: 600px) 650px, (max-width: 800px) 850px, (max-width: 1024px) 1024px, (max-width:1350px) 1400px, (max-width:1550px) 1600px, 2300px" alt="Alt Image"&gt;
  </code></pre>
  
  <p>The 1024 size is there, but then it jumps to the default size at 1920. 1920px is the full image size. I just can't seem to figure out why it's not pulling the two custom sizes. The images at those sizes exist. </p>
  
  <p>I've even tried uploading the image again to see if that was the problem.</p>
  
  <p><em>Note: I shorted the URL for the images.</em></p>