Can't mix my files!

[Cath]

Hi, am trying to overdub in separate mode. I record the playback track (using stereo - int mic) - that comes out fine. I then click on that track, go to overdub - separate - int mic and can hear the playback track as I record the next track. The problem is with the mixing. I go to the track I have just recorded, select mixdown, it does it's thing and then I can't hear the two tracks together. Where am I going wrong? Really appreciate any help! Cath