I'm sorry to hear you're experiencing issues with your tags. This can be very frustrating but I think I know what the problem is.
It could be that the volume of checkout events on your site is low.
The 'Add to Cart' tag needs to fire every couple of weeks and if you haven't had any checkouts Pinterest will flag your account with a warning that your tags are not ok and to fix it before you lose your merchant status.
You can remedy this by adding a $0.00 product somewhere on your site that doesn't get much traffic to manually trigger a checkout event yourself. If you have a free shipping code to use at checkout you should be able to successfully complete a 'purchase' without the need for payment.
This process should fire your checkout event codes and you should see the warning removed from your account within a couple of days.