c#无法找到已加载的程序集

I am writing an application which uses plugins. Plugins are class libraries which lie in Plug-ins directory. My app loads these libraries via LoadFrom. Some of them have dependencies in the form of libraries which lie in the same Plug-ins directory. When I try to create instance of class from one of plugins via Activator.CreateInstance i recieve an exception 'Unable to find assembly' (this is dependency assembly of plugin), but this assembly is already loaded (!) along with plugins and It is visible in ProcessExplorer. I can't uderstand in what my trouble is.I am writing an application which uses plugins.