Withdrawn post. I discovered many of my subs were not focused well and this is why they failed. When I eliminated the ones that didn't have good focus, all of the other subs registered successfully.
Why Do Subs Fail to Register in Pixinsight?
Well written Concise
ZWO 7X2 Filter Wheel Issue
The other night I was imaging remotely and when I awoke in the morning to inspect the subs, many subs were totally out of focus. The seeing wasn't good that night, but not that bad! I thought I may have a problem with the Sesto Senso 2 focuser. T...
Directly relevant; addresses focus issues causing failed subframes during imaging sessions.