So Peter, if I understand you are you saying that with a bad battery in the circuit vs a low charge battery one of those booster packs is not going to start it either?
Effectively, yes!
When you connect two batteries (or a flattish battery and a fully charged jump pack) in order to jump start a vehicle, the pair effectively 'become' one 'bigger battery', but the voltage/amp capacity across both rapidly
equalises down to something usually a bit above the voltage/amp capacity of the battery that started out with lowest level/state of charge. If your lower charge battery still has something in it, there's usually enough 'extra' juice in the paired combination to start the vehicle with the flatter battery; but if/when you've got a completely flat battery &/or one that has one or more dead cells in it, you're effectively connecting a good battery to a dead short, and that's not good for the better battery &/or either vehicle's charging & electrical system, and pretty much no matter what voltage shows from the remaining cells of the bad battery once it's connected to a jumping battery, a battery set-up either individually or as a jumped pair with a dead short in the mix still won't start your vehicle!
AND, if your 'bad battery' doesn't quite yet have any actual dead cells, just a cell or two (or maybe more) with a
really low state of charge, you might find that it won't even accept a charge if you try to charge it up with a normal charger, tender, &/or maintainer, because the majority of chargers need there to be
SOME voltage in the battery being charged in order to start the charging process. So unless you've got a charger that has a 'revive fully discharged battery' function, when you've connected up your completely flat battery properly and switch on your normal charger, it'll either reject the connection entirely and stop instantly; or it might show as charging momentarily and then either drop off or stop charging, possibly even revert to showing 'fully charged' immediately - but the battery still won't start zip!
