Why does the add to cart script work on some devices and not on others?

Question

JavaScript

Site milkyland.kz
There cost opencart 21 and the template is revolutionary. I am making this website for a client. There is such a problem: when the customer himself enters through smartphones or through a computer, the add to cart button on the product card does not work at first. Works only when you press the red button in the basket and back to the main. The icon is not displayed yet. Claims that he cleared the cache and tried from different devices.
I myself also did a test, it works perfectly for me in all devices! In addition to the us browser. it's a bit slow but still works.
Question. What might it depend on?
PS: click on the link at the beginning of the text and check and unsubscribe please, I want to know everything works for you or not!
Code snippets:
add to cart button

<div class="cart">
                  <a onclick="get_revpopup_cart(<?php echo $product['product_id']; ?>,'module',get_revpopup_cart_quantity_slider<?php echo $slider_id; ?>('<?php echo $product['product_id']; ?>'),'slider<?php echo $slider_id; ?>');" <?php if ($button_cart_class != 'prlistb active') { ?>data-toggle="tooltip" title="<?php echo $button_cart; ?>"<?php } ?>><i class="fa fa-shopping-basket fa-border" aria-hidden="true"><span class="<?php echo $button_cart_class; ?>"><?php echo $button_cart; ?></span></i></a>
                </div>

Answer 1

The icon is not displayed yet.

Such problems occur when a domain without www is specified in the config, and the user comes in from www and vice versa.

Answer 2

I suggest wrapping everything in quotes...