iTunes on iPhone

[buffymad]

I usually download music from iTunes on my iPhone (rather than from my computer) then add whatever songs to my playlist. Lately I go through the same steps but the songs don't appear in my playlist. The only way round it (well that I could think of!) was to delete the playlist, create a new one then copy all songs onto it. But that's a bit of a faff to do each time!

Any ideas what's going wrong or what I can try to put it right?