Mixing flooded batteries (your Group 24 starting batteries) with gelled batteries (your new Group 31 batteries) is not a good idea due to the differences in their needed voltages for proper charging
. (Note that mixing flooded batteries and AGM
batteries is generally OK, since they have similar voltage needs for charging
Moreover, the gels are more delicate than the flooded batteries. It's pretty crucial to get their charging and float voltages right and, especially, to avoid overcharging. If you treat them right, they can last a very long time, but they're also easy to kill with bad charging practices.
In the case you described, I'd speculate that either your cabling/connections are dirty or inadequate or the wet cell batteries are weak.
What you should do depends on your electrical
setup and your cruising plans. Since you've already got two new gelled batteries, you might consider buying
one more group 31 and using it for starting, while tossing the old flooded batteries. BTW, two starting batteries is overkill. You don't need them.
That way, at least you'd have all new batteries of the same type, requiring the same type of charging. Then, you'd need to be sure your battery charger and any other onboard charging devices can be set for gel cell charging.
PS: are you SURE you have gelled batteries? Many folks mix up gelled batteries and AGM batteries....they're not the same, and require different treatment.