FOP2 is quite simple on what it does, it fires a manager action to perform the requested command. If you have everything in place but still fails it might be because FOP2 buttons are misconfigured (with improper extensions and contexts), or Asterisk is misbehaving. Start FOP2 in debug mode, on the server side, and look at the events/actions/responses when you try to perform the transfer.
http://support.asternic.biz/?/selfhelp/view-article/how-to-debug-fop2/10
That will give you an idea of what is happening and why.
Best regards,