Ran into some similar nuisance flame failures, gas pressure volume issues and flame sensors were my problems. The gas volume one was a low pressure NG supply, when one boiler was running 100% and the cascade fired the second boiler, the gas volume just wasnt there and the pressure drop was something like 5 inches. Had to add some lockup regulators to appliances that couldn't handle the 14" on the line, but once we cranked it up the firing issues went away. Hopefully that lose connector was your problem, they are pretty hard to get off or back on sometimes!
Ran into some similar nuisance flame failures, gas pressure volume issues and flame sensors were my problems. The gas volume one was a low pressure NG supply, when one boiler was running 100% and the cascade fired the second boiler, the gas volume just wasnt there and the pressure drop was something like 5 inches. Had to add some lockup regulators to appliances that couldn't handle the 14" on the line, but once we cranked it up the firing issues went away. Hopefully that lose connector was your problem, they are pretty hard to get off or back on sometimes!
So far its still running without any issues, thanks for sharing your experience, I'll monitor the gas pressures if it fails to fire the next time.