Deflasked seedling die off

[Stray59]

Hello everyone and happy thanksgiving to the US members!
I have an issue - I have deflasked two flasks of seedlings and only a few (5-7) seedlings are alive; most dampened off. After the first batch died off and I had another flask that had to be moved out into the real world. After I planted the second, I decided to purchase Copper Soap, along with various other planting mediums in case I had better success with say rock wool over fine bark, coco fibers and spaghnum. I sprayed everything with copper soap, moved three 'strays' into the rock wool and two are fine, one is dead today. I have now lost about 2/3 of the last flask due to dampening off, so I am not getting ahead of it. Any suggestions will help. For those who have planted up flasks, what percentage of die-off do you consider "acceptable" or "expected"? I have increased air movement, sterilized everything, raised humidity, etc.
and, oh yeah, HELP - I have yet another flask on it's way! I really don't want to lose all of this one and am needing to 'catch on' to seedlings.