Most likely some of your files are locked. You can see if a file is locked by right clicking on it and selecting "get info". Just uncheck the "locked" checkbox on these files so that they can be deleted. The thing is that a folder or a subfolder can have such a locked file hidden in it, making the whole situation frustrating! So what I would do is download "superemptytrash" from :
http://www.faqintosh.com/risorse/en/sys/setrash/
empty the trash with administrator privileges and get it over with!
😀