Eggs being laid immediately before being set wouldn't affect fertility at all. Most likely, there is a hen or two that isn't being fertilized. The first time we set eggs from our flock (when we had only 1 rooster and 6 hens) I noticed that none of the eggs from a particular 2 hens were fertile, while all of the eggs from another 2 hens developed (the other 2 laid eggs that were so similar I couldn't accurately tell them apart and only about half of those eggs seemed to be fertile). Clearly some of my hens liked my rooster better than others!
We also candled again last night. The fourth questionable egg was without question dead, we could see what looked like veining but there was far too much light shining through the shell (it was a green egg, too dark to easily candle). But all of the others looked great! So now we have nothing to do but sit on our hands until Sunday. We're going to go into lockdown Sunday night, since we work all day and really only get over to check the eggs in the evenings, and also since the incubator has been running a little on the higher side and in the past we've consistently had chicks about a day early.