Why does my newly purchased song not transfer to my iPod?

This is happening over and over again, and it's making me crazy. I buy new songs on iTunes here and there. Every month or so, they seem to stop transferring to my iPod. If I look at the iPod through iTunes, it looks like the song is on the iPod -- the computer thinks it is -- but it never appears on the playlist on the iPod.

I don't think it's the song, because it plays on my computer.

I eventually give up on trying to make the song go to the iPod, and I reset the iPod, resulting in a long wait while it resets itself. This has happened several times in the last few months. That's ridiculous to have to do over and over again. Does anyone have any idea about what's happening and what else I can do besides resetting the darn thing?

After I reset it, it seems to behave itself for a while. I haven't updated anything since the last time I reset it, and it's been working fine, but now there's another song that I just bought that won't go to the iPod. So frustrated. Ideas?

mareneli