Just keep doing it. Over time your back and other muscles will loosen up and you should gain some inches. I was 14 inches away when I started and was able to get my lips around the head. Unfortunately, I couldn't get any further than that.
Do other exercises and don't focus strictly on one position. There are a lot of muscles and ligaments involved so you need to do work everything, just don't keep going if you feel pain because that means you're not doing something right.
The exercises I used to do were:
1. Sit in chair and bend forward and grab my wrist with my other hand under my thighs and pull down. This should work your lower back. If you try to really crunch and move your head near your crotch you will work your neck and shoulders. I could occasionally lick the head or wrap my lips around the head.
2. Sit on the floor, legs bent at the knees and your soles together. Grab your feet and bend forward. This should not only stretch your back but also your hips. Alternate this with the plow position which you are already doing. That should give a good workout.
3. I used to live in an apartment with a great bathtub where I could sit sideways with my back against the long side facing the rest of the bathroom and my feet up against the wall. While soaking in hot water to loosen the muscles, I could put the soles of my feet together and then grabbed my feet and lifted my legs over my head and back. Over time I was able to get nearly both my ankles behind my head. Unfortunately, when I moved, the new house had a regular-sized bathtub and it was too narrow to do this. I suppose if you're short you could still do it, but I'm 6'2" and it's impossible. I think that exercise worked the best for me because it loosened up my hamstrings. I just remodeled my bathroom but I couldn't fit a wider tub in so I can't continue this one.

You can also do this on the bed or floor but you won't have the support the sides of the tub give you.
Just keep in mind that I am not a professional so what I have described may not work for you and I take no responsiblity if you injure yourself doing this.