Add to cart not working properly. Always uses first item added.

I have really simple “add to cart” buttons on my page like this:
[wp_cart:Test Item 1:price:0.02:end]

I have the wordpress standard cart in my page sidebar. If I click on “add to cart” then it adds that product to the cart. If I then click on another product, it just adds the first product again. No matter what product I click on, it always adds the first product I clicked on.

This was working fine a few weeks ago?

https://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/