Umbraco 7 Custom Controllers not working

I have two umbraco sites, one on version 7.1.8 the other on 7.2.4. The site with 7.1.8 has several controllers all of which inherit from PluginController. Other than inheriting from PluginController there have been NO modifications to the models i.e. inheriting from Umbraco.Web.Models.RenderModel or in the view. It simply works just like normal asp.net MVC. The controllers have no reason to talk to Umbraco.

My second site however doesn't seem to want to work in the same way. Error messages insisting I used Umbraco.Web.Models.RenderModel etc.

Is there any way to tell Umbraco leave my controllers alone like in the first site?