Music syncing from iTunes to iPhone but not the other way around

Hi all,

Apologies if this has been covered in another discussion. I have a number of playlists on my iPhone 4 and I continually add songs to these playlists. Now normally, when you sync the iPhone to iTunes, all these playlists should automatically update in iTunes also. However, what's happening with me recently is that every time I sync, the playlists on my phone revert back to the original playlists on iTunes, so if I add 10 songs to a playlist from my phone, after the sync, those 10 songs are no longer there as they have been replaced with the songs in the original iTunes library. Extremely frustrating as I don't want to have to manually add music to my phone playlists every time!

Hope this all makes sense! Can anyone help please? It seems trivial but it's driving me mad!!!