The checkout process is not functioning correctly after updating the cart.

Please help!
Why is it that when I add a product to the cart, and then refresh the cart (for example, after adding more items), I get redirected to the homepage after clicking the Check out button? I have to go back to the cart again to continue the checkout process. However, if I don't update the quantity, everything works fine, and the checkout process continues.
I'm using Drupal 7 with Drupal Commerce for the cart.
Thank you to everyone who took the time to respond!