I have 2 incubators, 1 homemade and the other was bought. Both have great hatch rates. I’m actually going to be making a bigger one this winter.Do you have a incubator homemade or an actual bator??? With both i have late hatches as does my friend and we have to assist them with pinsize hole where air cell is. Something isnt right!
I’ve only ever assisted with a malpositioned bantam egg and that was because he couldn’t zip out.
I think low temp is your problem and maybe humidity as well. Its difficult to get it just right starting out.