You can’t publish this draft because another user has removed your lock on the file

I currently use dreamweaver to create all my websites.  My client uses contribue to updated them.  She can edit some pages, but then when she gets to other pages she gets this error:

“You can’t publish this draft because another user has removed your lock on the file. Select File>Actions>Publish as New Page to publish your draft with a different filename, or click Cancel to discard the draft.”

I can't figure it out, all the permissions are the same.  Why is it happening in one folder and not another folder?  And how can I get this fixed?

Any help is appreciated.

Thanks Jacey