What exact steps are you using to open the file? If you choose Open Remote from a problem computer, can you see the file?
If not, check Edit | Preferences | Application on the client computers and make sure that they have been set up for the same network protocol. (Back in the ancient FMP 4 and 5 days, the fmp app supported more than one network protocol.)
Next check the network properties for the network protocol you've selected on a working and non-working computer. Make any changes to the the settings for the problem computer needed to match that set up for the working computer.