Having spotting and breakthrough bleeding is normal while on any pill, it's not unusual. In fact when I was on Desogen, I remember having that problem a lot, so that's why I ended up switching. I think at that time, if I remember correctly, I was on Desogen, switched to Ortho-Tri-Cyclen and then to Microgestin, which I stayed on the longest. I had breakthrough bleeding sometimes that lasted a few days on microgestin also, but not as bad as I did on Desogen (or Demulen also when I was on it.).
If it gets really bothersome you will probably have to switch pills. I had to switch so many times while I was on the pill that I lost track. But eventually I just gave up and quit taking the pill all together because it wasn't worth it for me anymore.