Hey chile-pepper,
Make sure all your plugins are updated. If it’s still not working I would recommend disabling all of your other plugins temporarily and see if the Testimonials plugin starts working again (I know it isn’t a fun process but it’s the only way). A quick way of doing this is if you have cpanel access you can create a new directory in your wp-content folder (I usually call it disabled-plugins) and move all the plugins you want to deactivate into this folder. They won’t run and as long as you don’t load your plugins page in wp-admin they will activate again automatically when you move them back into the plugin folder. If you do visit your plugins page in admin while they are in the other folder they will disable!
Once you have disabled all your plugins and verified the Testimonials plugin is working re-enable your plugins one by one till it stops working. Then you will know where your conflict is.
If Testimonials still doesn’t work I would also try switching your theme to one of the WordPress default themes like Twenty Fifteen or Twenty Sixteen. I’d do this with the plugins still disabled.
You can also take a look in your console log and see if you are getting any JS errors in your browser. If you like you can provide a link to your site and I will take a look.
Hope this helps,
AJ
