I also experienced this issue recently. In my case, it happened that I had included fitVids.js in my theme and then uploaded a plugin which also included it. Commenting out the line in my functions.php where I had enqueued the fitVids script then resolved the issue, and the buttons worked as they should.
I'd suggest going through and disabling plugins one at a time in your theme to see if any of these are causing the conflict here, and also commenting out any additional scripts you are enqueueing in your functions.php one by one to try and isolate the issue. You may also be calling scripts in your header.php so try disabling/re-enabling them too.
Hope this helps in some way