Coding issue in our theme somewhere

Hi! I have a shopify store with the theme "Ella", and recently we have been having issues with people ordering our t-shirts. Anytime they select a size other than "XS", our site somehow automatically reverts it back to "XS". (Example: someone selects our tee in a size "L" and adds it to their cart. When they go to checkout, it says that they have selected a size "XS"). We tried contacting our developer about it and they weren't helpful at all. We need someone who can locate the problem and fix it asap. Thanks!