So, basic battery maintenance 101 asks, do you have sealed batteries or conventional wet cell with removable tops? Has the battery water been checked? Has a hydrometer been run thru the batteries checking for a bad cell? Cables are clean?
Now, battery charging 101. Your battery doesn't charge instantly. If you have a low battery it charges back up depending on a couple of main items, one charger capacity and two state of charge.
Charger capacity. I'm going to use the house batteries as an example here, but your chassis batteries are similar. House batteries going dead and "not charging" as expected is the more common problem.
Some inverter/chargers can put as much as 150 amp/hours back into a battery. Many smaller plug in chargers charge as little as 2 amp/hours back into a battery. Most 6 volt deep cycle batteries have 200-240 amp/hours when fully charged. Depending on how discharged the battery is, do the math and see that it could take days with a 2 amp charger.
State of charge. As batteries near full charge they accept a charge more slowly. Hence the "three stage" chargers you may have heard about. Your meter may be broken down into three stages called Bulk, Accept and Float, or similar descriptions.
So here's something that MAY be happening in the OP's battery system. A bad cell in one battery is shorting out (low water, sulphation, age, vibration, etc.) this acts as a drain on the entire system slowly drawing down the state of charge. He jump starts the MH and charges the battery at Bulk rate. Here's the math, 230 amp/hours full charge battery, if he has maybe 25% of capacity left, he needs about 160 amp/hours put back in. Charges for 2 hours at 25 amp/hours. In reality put only 50 amp/hours back into battery that needed 160, so still 110 amp/hours low. Have we got aha moment yet?
From the descriptions I'll bet there's a bad battery cell causing the problem.