I guess this is a really common problem. I had the same problem but was sat in bed one night and had the guitar to hand and played as suggested in the (excellent) video, couldn't quite get it, subtly shifted my finger and, again, couldn't quite get it, subtly shifted again... trying to find a position that sounded better. After probably 15 minutes of doing that, I found the perfect finger position and then spent the next 30 minutes changing between a-minor and F-major.

I'm not quite sure the point of this post. I think I'm trying to say to keep trying very subtle variations and you'll eventually get it and then you'll need to find ways to repeatadly get it. The second part is probably harder than the first but worth persevering worth.