HI,
We haven’t made any changes to the plugin in almost a year. It’s quite difficult to say why that particular sidebar isn’t working, but if it was working before and you updated the theme in the meanwhile or any plugins that could be affecting the theme or the sidebars then try getting the old versions back.
Please make sure you have backups before doing so, or even better, do everything on a staging copy of the site.
The problem is that the custom sidebar option on the menu of an especific page is the only one that doesn´t appear.
Finally I´ve installed another plugin that works fine: WooSiderbars