The majority of people interpret the fax machine as an annoyance, and many dream about the machine because it is an important message.
A fax can be seen as a business signifier, and a difficult situation that needs to be solved. And when someone feels ignored by somebody important in their life, this may appear in their dreams as a sign that they need to call and make contact.
Also, in the context of a dream, a fax machine can represent the intake of information and its subsequent processing. It can also signify a sense of urgency or panic. When it comes to the causes, these may depend on what you were doing before you dreamed about trying to use a fax machine. If you were filing paperwork with one before going to bed then your dream may show that as a means to process your emotions or thoughts in that moment. If you were sending faxes, then it may be showing hesitation in making decisions or trying to attain goals.