Disappearing Playlists

I really can't take this anymore - it's been going on for months (possibly a year) and every time I speak to anyone else, they tell me they simply don't use iTunes any more...

Anyway - after I create a playlist on my iPhone, when I plug it into my iMac, pretty much all the playlist disappears - down to one or two songs. Everything is up-to-date, as I keep updating in the hope that this time it will get sorted. The whole reason I have an iPhone is so it can sync easily with my computer, but if it's not working, then it's pointless and I may as well get a much cheaper phone.

Does anyone have a solution that doesn't involve the word 'Spotify'?