Actually this topic does not belong here but i dont have permission to write it to more related categories. Thanks to roblox’s rank system!
I tryed to use workspace:Raycast method to detect some parts on the ground but i couldnt achieve it and couldnt figure it out for 6 hours(!). And voila the problem was i used another collision group for part which has not collision with Default. So when i marked collision with Default the problem has gone. I read documentation 2 times but i never saw something like that. I think this is not intended behaviour but i need explanation.