Update Custom Object between different users

I have a problem with the update of the custom object (CO) between different users. L 'userA creates a CO. Acls associate with a public reader = true, writer and public = true. When the userB try to update the CO gives me the following error: "you do not permission to update this object".

I also created an alternative way to handle the problem but the result is the same. I associate a private ACLS and add the user to write and read the UserB. When the userB try to update the CO always gives me the same error: "you do not permission to update this object".

I'm desperate can you help? Thank you. Gianfranco