Its not confined that it just issue of js or anything else, so you can check whole way that you used for implementation, and then let me know if cant work.
I've double checked all of the steps and have implemented them all! Here's the preview link from my old theme page where it works if this helps?
As i can see in preview theme popup is working by light box Js and on your new theme its not working fine with size chart, may be you have to not initialize it in you Js, so check it by Js
That's i cant say anything without checking any code. you have to follow this instruction and check what you missed or it may be you have error in Js(theme.js) file
May be its following error that's why size chart not working? you have to solve it and then try.
Sorry but without checking code i cant send you any exact solution for js issue.
Add me as staff (email@example.com) i will check code and try to solve.