Without having admin access to your site, there isn’t much I can do for you other than a couple of other things to look at:
1. Make sure you are using MY plugin – PTA Volunteer Sign Up Sheets plugin and NOT the older Sign Up Sheets plugin from DLS software. They are Not compatible and use different shortcodes.
2. If you are 100% sure you have it all setup correctly, then it most likely is a conflict with some other plugin or theme. Something is breaking something in the background before the shortcode can be processed. You will most likely need to do a process of elimination to figure out what it is. A good place to start would be to look at any type of Visual Page Builder type plugins that try to help you make cool looking pages without knowing anything about html… those are mostly overly bloated tools that often cause issues with other plugins. The plugin is still working just fine on my site with the latest version of WordPress, and there are over 2000 active installations, and nobody else is having this very basic issue. That shortcode drives the whole plugin output on the public side, so, of course, I would have at least that bit working correctly, or would be hearing from thousands of users if I somehow broke it.
Finally, I’m closing this thread for 2 reasons:
1. This is NOT the support forum for the free plugins. Those are on the WordPress.org site, as I have mentioned all over the place in sticky “PLEASE READ” notes on these forums.
2. This is a problem with your site, and not with my plugin. If you want to provide me with admin access and troubleshoot for you to find the issue, then my rate is $75 for up to one hour of work. You can contact me privately through the contact form to set that up. I don’t provide that type of support for free plugins, as it’s clearly not an issue with my plugin.