For me, this has been a problem for which there is no quick fix - the reasons for becoming stuck and "unable" to finish a track are actually quite diverse. Overall, however, I believe the reason is simply a mismatch of intention and the necessary skills. Here are a few things I've done that have significantly improved my ability to finish tracks:
1) Practice Writing - Do your arrangement first with all basic saw waves. Load a project with 5-6 tracks on a saw wave, one drum rack with only a basic kick, snare, hihat, and crash, and just write a song. This separates out the process of thinking of all the different notes/parts of the song and how they fit together as a whole. Then produce it how you normally would. Do this about 2-3 times, and you will find your brain expanded in lots of interesting ways when you go to produce your next track. Mainly, you start hearing potential, and brainstorming what to do about that potential as you go. This keeps you moving and active - vitally important for the production process.
2) Practice Sound Design - Intensely study one sound design technique, and commit to repeating and mastering it 10-20 times. For me, this was creating FM basses with resampling. I following a resampling tutorial and creating 15 bass sounds in 2-3 sittings. Don't stop until you hit your target and you are finding things trivial - the creativity comes when you master things that would previously get in your way. Music producers don't utilize enough repetition for the sake of sharpening skills and allowing the music to flow out.
3) Practice putting it all together, in a specific/structured way. Try the subtractive arrangement workflow. This is better explained on EDMprod.com, but basically you create a dense loop of musical elements that constitute your "main" section, copy it across your entire project to span about 5 minutes, then arrange by deactivating clips. As far as adding things in later, there are a few rules for this system that you should read about on EDMprod (
http://edmprod.com/subtractive-music-production/). Do this one or two times faithfully.
If you do the above, you will approach producing a track with an elevated sense of intentionality and confidence, and finishing tracks will seem (almost) trivial. You just need to brush up on some core metaskills!
Hope this helps,
~Zildy