Sounds like some javascript is going failing, possible a plugin conflict.
Did you change your theme or install other plugins between the time it was working and it was not working? Try to deactivate those for a moment, see if it’s working and then activate them one by one and check. At the one it stops working, that’s the problematic one ( please let us know which so we can check as well ).
The plugin Oypo conflicts with Maxbuttons
I tested the plugin and so far I didn’t see any problems. Please describe what the issue is exactly.
Ah I stumbled onto the issue with the button editor and this plugin. Please file a bug report with them. They shouldn’t overwrite the WordPress default styling, which is what they are doing, and keep to plugin guidelines.
In general, if you can help it I would not recommend running a plugin like this, since it’s very poorly made.