Not answering a call is different from a failed fax, either it picks up the line and attempts to receive the fax and fails, or does not pick up the line at all. If it doesn’t pick up the line (no answer), then this is a problem specific to the other fax machine you are calling.
If you can send to every other machine, then the problem is specific with the fax machine you are sending to. What type of fax machine are you sending to? Is it connected to a voice answering system? Have you tried calling it with a regular phone to see how it answers a call?
Changing speed, compression etc. doesn’t effect sending unless it actually makes a connection with another fax machine.
You will have to provide more details, such as the type of fax modem you are using , the version of WinFax, version of Windows before we can provide additional assistance.