During app installation it adds some code to your Theme's templates. If you installed new theme after the app, it will miss required code and app will not work.
The easiest way to fix that is to go to Apps -> EasySlide -> Installation and activate the accordion using the instructions.
If it doesn't help or you have any difficulties, you may grant our support team access to your Store Admin - we'll add required code to your templates manually, and make sure app is working properly. You may send invitation to support@nexusmedia-ua.com. Please don't forget to send us an email explaining your problem.