The 18 degrees C (64 F) inside the house was a great temperature for storing them, just barely on the warmer side of perfect. Wish I had a place that cool to store mine. I did not read that quite the right way. I was not sure if it was 18 C in your house or 18 F in the hen house. You said you did not keep them that long, so storing them was not the problem. You don't even have to turn them for the first few days. From the way you posted, I think these are eggs from your own hens and rooster.
With 6 out of 9 or 10 not even starting to develop, it sounds like a big part is a fertility problem. I have not hatched in the winter, so I don't have any first hand experience, but some people have posted that they experience reduced fertility in hatching eggs in the winter. With chickens being chickens and not consistent at all, others on this forum have said they get great hatches in the winter. I don't know if winter has anything to do with your situation or not.
How many roosters do you have with how many hens? There are a lot of myths floating around on this site about hen to rooster ratios and fertility. The ratio commercial operations use in their specific situation is not a requirement for all of us, but it is still possible you don't have enough roosters for your hens. And a whole lot depends on the specific rooster. Some are just a lot more vigorous than others.
Age of your flock can be a problem. Older hens can lose fertility. I'm talking five or six years old, not two or three years old. A rooster also can lose vigor and vitality as he gets older. In my post, please read "can" to mean it might happen. "Can" does not mean it does happen each and every time. You'd be surprised how many people on this forum have trouble understanding that.
If chickens are inbred for several generations, they can lose fertility. There are techniques to avoid that, but for most of us with small backyard flocks this is one reason to bring in fresh blood every few years. We need to keep that genetic diversity up.
Some thick feathered chickens, like Orpington or Cochin, can have feathers so thick the rooster has trouble hitting the target. This is not always a problem, but some breeders trim the feathers around the vent to give the rooster a better target.
Is there the possibility some eggs froze before you gathered them?
As for the three that started but stopped, there are many reasons that could have happened. It is possible the hen did something wrong during incubation. Most hens will do a good job of keeping the eggs warm even in really cold weather. Most do a great job, but they are living animals so anything can happen. Since being broody is purely instinct, I think it is surprising how many actually get it right their first time.
You might look through the links I gave you to try to figure out what happened. Those are set up for commercial operations in an incubator more than a backyard flock under a broody, but you can maybe get some insight as to what went wrong. But sometimes we just get bad hatches and can't figure out what went wrong.
I don't know if this helps you any or not. Basically, it could be a lot of things and I don't know what it is. I wish you better luck on your next hatch.