TASCAM 424

[Sledgehammer]

I've just bought an older 424 not an mk. I've only begun to explore this machine. I seem to be having a problem recording on tracks 2 & 3 during an overdub session. Can someone advise me what's going on. I started by recording a drum maching to track 1. No problem!! Then a guitar to track 2. No problem!! But, when I tried to do a vocal on track 3, I could not hear track 2 playing in the phones only the drum machine part from track 2. And lo and behold the guitar part was erased. I tried again, thinking there might be something wrong with track 2 or 3, this time I recorded the drum machine to track 2 and the guitar to track 3 at the same time. Then when I did the overdubs to tracks 1 and then track 4, I was then able to hear the other tracks playing just fine. What's Going On? HELP!
If it matters, I was not recording with the direct feature at first but tried later and it did the same. email me with your advise, thanks.