If there is something wrong with my incubator, then all the eggs should have problems, not just a few, right? And it would be early on, not on day 49?

I have 37 eggs in there now, I noticed 5 in the same box of 7, the first clutch this year and furthest along, had 4 eggs looking odd a few days ago. Today they are a little worse, patches of transparency and slightly green. I candled them. The 4 that looked bad have no veins. The three that look good, only 2 has veins. I presume that means 5 are dead.

All the other egg boxes look great candled with veins. Even the ones which I were worried about before in another post, because they looked dry when discovered, has plumped up.

Yes, pics I know. Later. I am too depressed.

There is two probes in the incubator top and bottom. Always same temps.

They are also on the middle level. All eggs above and below are fine, though not as far along day wise.

I can't remember if I candled them in the first place if they were fertile or not.

Is there any incubator problem that can cause death in eggs only so far along and not before?