Only just found this thread, but when discussing students who fail to go solo after more than 40 hours, perhaps you'd benefit from hearing from someone who was once in that position, ie ME!
I started f/w flying before I ever discovered helicopters, for those who don't know me. I wasn't a natural pilot, but I suspect that without things going wrong I would have been an average student. But I had an instructor I didn't get on with, got very scared quite early on while trying to land, and after that lost confidence completely. Being stubborn, I gritted my teeth and stuck with that instructor for quite a few more hours, until the school suggested changing as I was getting nowhere. I was also so lacking in confidence by that time that I could barely drive to the flying school, never mind fly!
My new instructor now had a very, very difficult job. He wasn't just trying to teach me to land, he had to deal with someone with severe lack of confidence problems. It's a very different skill, and not one they teach you on FI courses, or they didn't on mine anyway (some years later of course!). But anyway, he talked a lot, and tried to calm me down, but it didn't really work. Looking back, I think he was a very good instructor, but not for me at the time.
I eventually got an instructor as stubborn as I was, but also very calm and phlegmatic. He simply kept at it. He didn't do anything special, or say a lot, but he never gave up on me, and he never got upset or acted as though I had a problem. It wouldn't have worked for everyone. He was a lowish hours hour builder, but his personality and approach happened to be what I needed at the time. He eventually sent me solo...after a total of 48 hours!
The point I'm rather longwindedly making is that this 'good' v 'bad' instructor thing simply isn't that simple when you're dealing with individuals and something as complex as learning to fly.