Invoice not accepting payments at checkout

My site randomly stopped accepting payments at checkout and I’m not sure why. I’ve had a developer look at the code and he can’t find anything wrong with it; he’s ensured that the WooCommerce PayPal API information has been set up correctly, and that the checkout process is using your Invoicing plugin. Within the Invoicing plugin there doesn’t seem to be a place to set the PayPal API information, so I’m wondering if that’s the issue. I’ve ensured that the shortcode is on the checkout page but each time that I go to checkout I get an error from PayPal saying that something went wrong. Please help!