The soft-fax facility is 99% reliable. Sadly, it is still only a piece of software pretending to be a fax and is ulikely to ever be 100% reliable.
We are constantly monitoring progress as this software improves, so if you have issues, let us know and we can keep you informed if things improve.
It is also possible that the Fax receive is working, but the email sending is not. If this is the case, you can find a copy of the received fax on the Users->Messages screen if logged in as administrator. Email configuration is available on the System->Global->Email screen.