Itunes on computer not syncing with iPhone

I've had 2 long conversations with Apple Support about this now and the problem's still not solved. Since I got a new MacBook, my iTunes on my iPhone won't sync with iTunes on my laptop properly. Now, when I create a playlist on my iPhone and sync it with my computer, half the playlist disappears on my iPhone or the playlist disappears entirely! What has happened to iTunes? It's not as foolproof as it used to be. In the old days, I could search for playlists, songs etc easily on iTunes now the options have changed and when I pressed sync, it used to be exactly the same on my iphone as my computer....Can anyone help please? I don't want to spend another hour on the phone with Apple.