Local Files not Syncing

[Streamlyne]

Local tracks aren't syncing to my device at all. Tracks will sync if they're available on Spotify's database, but if they're local files on my computer, they don't sync. 
The device doesn't show up at all in Devices on the desktop version of Spotify.

This is an issue i've had with Spotify since I first signed up last year. 

The album appears on my phone under "Playlists" but under the title, it says ""0 of 15 tracks synced" or "1 of 15 tracks synced". Sometimes it will sync a few tracks (i'm assuming because it's available through Spotify's database) but the rest do not sync. 

They're playable through the PC version of Spotify, but won't sync to my phone. 

This only happens every now and then. Some albums are perfectly fine, but some aren't. 

I have an iPhone 5. 
I'm running Windows 8, and the latest versions of Spotify for PC and iPhone. 
I've tried restarting the internet connections, restarting the devices etc - to no avail. 
I've removed the playlists, and created them again - same thing occurs. 
I've checked every thread on here that sounds similar to this, and nothing there seems to help. 
I have no firewalls etc blocking this from happening.
I've tried running the program as an administrator.

Can anyone help?

[Streamlyne]

Nobody?

[Adam]

You might already have checked but: does your router have UPnP enabled? If so, try disabling then re-enabling it.

Feel free to also get in touch here and our support team will get back to you as soon as they can. If you get an automated response, reply back to it directly.