Why is my Shopify cart page not updating quantity when a user clicks the plus button?

Quantity update failures happen when AJAX cart scripts fail to send the quantity payload to Shopify’s cart api, or when your theme’s JS conflicts with automated discount calculations.

“This tends to crop up when the AJAX request isn’t actually sending the new quantity to Shopify, or a discount script is quietly stepping on it. I’ll trace the request and have it updating properly within 24 hours for $99.”
— Jeff Bollettino, Managing Director, Ignite Conversions