Always good practice to test after building. WebGL has some differences that can break things that work in the editor. Hopefully you can update it after the jam so people can play!
I typically only find answers on an issue specific basis, but here are the Unity pages giving an overview and what the platform does/does not support, and then the WebGL debugging page. For you specific issue I would also do the standard google search which should be able to help out too, since it seems like a lot of people have issues with WebGL builds/builds in general.