^^^This is what I saying in post #5. Sorry I never elaborated. Try this:
1. Remove all the plastic that is remove-able.
2. You will see 2 pawls, one that makes the shifter pull the cable and one that releases it. Check whether the pawls completely engage the teeth.
3. Move the faulty pawl in and out to release the gunk that is holding it back. Use a pick or small screwdriver. Also clean the ratchet teeth.
4. Spray with degreaser.
5. Blow with air-compressor if you have one, or rinse with water.
6. Repeat 3-5 until it starts to consistently engage.
7. Lube with light oil, silicone or teflon.
8. Re-install