I assume you have selected wav in the drop-down of the Export options.
A few thoughts:
• If you are using external synths that are not recorded as audio and/or soft synths that are not frozen, you need to check INCLUDE LIVE AUDIO in the Export box or you won't get any sound.
• There is a quirk sometimes that select all doesn't actually select the tracks when you use SELECT ALL. As a work-around, I control-click a few tracks first and then go to SELECT ALL. I don't know why, but this seems to fix it. Once in a while, I create a wav or mp3 and then find that the bass or another track didn't get included.
• If the process is taking a long time, I wonder if you have something you don't know way out at the end of your track. Selecting INCLUDE LIVE AUDIO, the process should take no longer than the length of the song, plus a few seconds of processing. Maybe try this: After doing the SELECT ALL, put your cursor where you want the song to start and go to Edit | Select | From = Now. Then put your cursor at the end of the song and go to Edit | Select | Thru = Now. This will eliminate the possibility that you have an hour or something of blank space at the end of your project.
• You can always try selecting a small section of a single track as a test.