Toenail damage and halted growth

For about 4 years I've had damage to my toenail. It is thick and yellowed. I originally thought it was fungus and was treating it with an OTC anti-fungal liquid, but after this hadn't had any effect for a couple years, I visited a doctor to get a professional opinion. The doctor sent clippings to be tested and the results were that there was no fungus. He instead said that I probably just damaged the nail and it needs time to grow out.
Recently it started to seem like the nail was finally healing (healthy, pink nail was growing in and pushing out the yellowed section). But a few weeks ago, I noticed that the nail had stopped growing. I noticed this because I had polished my nails two weeks prior, and when I was removing the polish I saw that the line of polish was above my cuticle on all my other nails because they had grown, but on my big toe the polish line was still exactly at my cuticle where I had painted it - the nail had not grown at all in two weeks. I have since kept them unpolished to monitor my nail's growth, but it still seems like it hasn't grown at all. For about a week I've also been applying Sally Hansen Nail Growth Activator, but that doesn't seem to be helping either.
Why has my nail suddenly stopped growing and what can I do to help it grow again?