Hi guys, make sure you have plenty of free space on your devices (500MB+ minimum) and you haven't already hit the 3,333 track limit. They're the most common causes of syncs stopping midway through a playlist.
Deleting playlists doesn't free up any space. The only way to clear the spotify cache (I believe this is a feature of iOS) is to remove and reinstall spotify. Make sure you have 500MB or more free before reinstalling.
I was having trouble syncing some of my playlists to play offline too. For some reason some playlists would sync simply by pressing the "make available offline" button on my iphone, others wouldn't, the "make available offline" switch would simply switch back to off every time.
Try this: get onto spotify on your computer, select "devices" under the left hand menu. Make sure the "available offline" button is switched off on your iphone/ipod/device. Check the box for your playlist under the "device" tab. Sync should start automatically.