Add to Cart not working in some browsers - VirtueMart joomla

I have a website, where vm and joomla are customized. I recently had a problem that customers can not add products to the shopping cart in certain computers and mobile devices. Sometimes it works and sometimes it does not work. Als,o the administrator and the customer don’t get any confirmation email from vm. ( no invoice ) I now have to manually confirm the orders every time. and when I manually confirm the order in the backend, I get an error: vmError: Warning extension of the image is a png, tpcdf has problems with that in the header, choose a jpg or gif. I found out that the problem comes through the ajax. Because AJAX is not running properly and you will not be able to add the product to cart. i have disabled Ajax and now it works but you do not see a popup anymore and you are immediately redirected to the shopping cart. I am looking for a freelancer who is a good programmer and who has a lot of experience with joomla. So that this problem can be solved. I have 4 websites if you can solve this problem properly then you will become my regular freelancer for all my projects and websites.