Dropbox for Business user here. @willofmaineIt sounds like you're on the right track with ensuring that the folder path in Dropbox for the VWXP is set to 'Make Available Offline'. This is key and it can trip-up users who started using Dropbox with file storage set to Offline-only. If you had it set to Offline-only and changed the default to Available Offline, right-click on the root Dropbox folder and select 'Make Available offline'. I've sometimes found that the folder availability isn't always inherited and double checking this can help.
I also work in an environment with project sharing users scattered around the world, both employees and independent contractors, working on the same file. Let’s face it, project sharing isn’t a technically simple platform; there’s a lot going on in the background the make it function. It’s pretty much an enterprise-level service so communication is important; especially with independent contractors. There are a couple of scenarios where I've seen VWXP's go into hiding:
When the VWXW is stored on Dropbox
You'd be surprised how many users store their VWXW file in Dropbox, so have the user check this first. If this is the case, have them 'Save a Copy As' a regular VWX file somewhere else. Then, delete the existing VWXW and have them create a new VWXW from the VWXP and save to a very simple, straightforward location like C:/VW Project Sharing.
When the VWXW resides in a folder being synched by a different service
In addition to ensuring that the VWXW isn't on Dropbox, also ensure that each user's VWXW doesn't sit in a location where any other synch services are running. This includes OneDrive, Google Drive and any proprietary sync service, especially on Windows. If you discover this, again, have them recreate the VWXW and save it to a local folder with a short file path. I don't think you'll find this one written in any official VW documentation, but I can assure you that it's an issue and that once resolved, Project Sharing will stabilize.
Irrespective of which Dropbox or Vectorworks version you're using, here's some unsolicited tips from my 'Lessons Learned' notes for Project Sharing:
Ideally, all users would be on the most current version of VW.
If this isn't possible, ensure all Project Sharing users are on the same VW version and the exact same Service Pack
Ensure all users are on the same version of Dropbox (EXACTLY the same) - right down to the very last version decimal place!
Save and Commit at predetermined times so that users aren't synching to the VWXP at the same time
Proactively clear the projects sharing metadata
Proactively recreate the VWXP from time-to-time
Inform the entire VW Project Sharing team before recreating the VWXP file
Don't Save and Commit and close your laptop lid. Ensure that both the VW synch and Dropbox synch have finished before putting your computer to sleep and having a Friday beer.
Speaking of Friday beers...cheers!