For those having tough hatches, I was helping troubleshoot with a local friend that lost several chicks after lockdown. We came up with a few possible (probable) issues, so I thought I'd share.
When everything else seems right, look at the simple stuff.
-Sterilize your incubators between hatches.
-Use rubber shelf liner on the bottom, not towels, paper towels, etc.
-Leave vents open!