ACF outputs html but not fields

I have acf setup to output a repeater field of things.

Here’s my code.

<?php
$rows = get_field('hero_slider');

if($rows)
   {
      $content = '<ul class="ei-slider-large">';
      $slider = ' <!-- ei-slider-large -->
                       <ul class="ei-slider-thumbs">
			    <li class="ei-slider-element">Current</li>';

       $count = 0;
        foreach($rows as $row)

	{

	$content .= '<li>
		            <img src="' . the_sub_field('slider_image') . '" alt="' . the_sub_field('slider_image') . '">
			     <div class="ei-title">
			       <h2><span class="blue">' . the_sub_field('slider_title') . '</h2>
			        <p>' . the_sub_field('slider_description') . '</p>
				<a href="' . the_sub_field('slider_link') . '" class="emboss_button">Learn More</a>
			       </div>
			      </li>';

	$slider .= '<li>
		          <a href="#">Slide ' . ++$count . '</a>
			 </li>';

				}

	$content .= '</ul>';
        echo $content;

        $slider .= '</ul>';
        echo $slider;
			}
?>

html outputs but the content inside of those fields dont.

I have a stuck “loading” on the page.

Any suggestions would be greatly appreciated! I’m assuming I have written something wrong that goes along with acf. – just can’t for the life of me put a finger on it.