In Bold Motivator Premium, part of the liquid code install will disable the quantity box for the free item. If a customer can change the quantity of the free item in the cart, the Motivator's liquid code isn't installed or implemented properly.
Visit Bold Motivator Code Install Instructions - Premium for more information on how to set it up.
Your site may also be using Ajax functionalities that update your cart information dynamically. Ajax is a series of web coding techniques that allow for information to be updated on a site without having to reload the page.
Sometimes, this functionality can cause issues with how Motivator catches changes that are made to your customer's cart and will allow them to adjust their cart quantities.
We generally recommend either disabling your theme's Ajax functionality, or contacting our Merchant Success team to see if we're able to allow Motivator to work with your Ajax cart.