Do you have the dropbox app on every computer, so that the files can be accessed through finder? I use Google Drive to sync phenix project directories between my 3 Macs, and usually it works perfectly. If I run a job on one computer, I can open a phenix gui on another, and it will show the intact job history, including jobs run on a different computer. Sometimes I get a error that 'this is not a valid phenix project directory' when I attempt to import a job folder created on a different computer. When that happens, I just make a new sub-folder within it, make that the new project folder, and move on...so maybe I'm running into the same problem as you and ignoring it. Usually it's problem free however.
Nat