Some project tracks failing to bounce with the others

I recorded an audio and a software track simultaneosly (vocal & piano). I bounced them to an mp3 & aiff file and they sounded fine. Later I added two new software tracks to the project. Playback of all 4 tracks sounds fine; however,when I bounced the total project again (now an audio and 3 software tracks) it only captured the 2 original tracks. If I mute the 2 original tracks, the new ones will bounce fine, but all 4 will not bounce together. What is happening here? I'm wondering if this somehow relates to the fact that the 2 original tracks were recorded simultaneously, but not sure? Is there an easy way to work around this??