Can't extend CI_Controller

[eluser]Lukas1[/eluser]
Hello, I am using CodeIgniter 2.0 and I am new to the framework. I tried extending the CI_Controller class and it works for me on my computer using Wamp. But when I upload my project on other servers, I get this fatal error:
Fatal error: Class 'MY_Controller' not found in [PATH]

I don't understand why is this happening, my extended controller is in /core folder(not /libraries), the prefix is correct and it works on my computer, so I don't know, why it doesn't work on other servers(I tried 2 different servers). Does anyone have any idea, what could be my problem?