Original: The jar certainly serves as a prison for the evils that Pandora released - they only affect mankind once outside the jar. Some have argued that logic dictates, therefore, that the jar acts as a prison for Elpis as well, withholding it from men.
Simplified: Elpis is everything that remains in the jar, when Pandora closed it again, so does the jar give elpis to mankind, or does it keep elpis away from it?