Another thought, is your product price greater than 0 (zero)? As price is not displayed if price is zero. This could be an issue with your pricing or if your zen_get_products_display_price is not is not working as expected.

First check to make sure you have the version of /includes/templates/bootstrap/templates/tpl_product_info_display.php from bootstrap 3.7.1

check if that fixes things

if not then if you add the line below in before the if statement at about line 238 it should display the values for display price bottom and the price.

Code:
echo '<p> display_price_bottom is ' . ($display_price_bottom ? 'true' : 'false' . ' display price ') . zen_get_products_display_price((int)$_GET['products_id']);
Thus
Code:
<?php
}
echo '<p> display_price_bottom is ' . ($display_price_bottom ? 'true' : 'false' . ' display price ') . zen_get_products_display_price((int)$_GET['products_id']);
if ($display_price_bottom === true && zen_get_products_display_price((int)$_GET['products_id']) > 0) {
?>
            <!--bof products price bottom card-->
you should get a line output that looks like this
Code:
display_price_bottom is true display price £27.00
if display price bottom is false or the price is £0.00 then we are a step closer to finding your issue.
if noting is displayed then there is something else wrong.