A few things to consider:
Mating up batteries of different ages and/or life cycle conditions isn't really advisable, but if you are having fairly even battery voltages under load testing, you might want to overlook that for now. The alternative is buying an entire new battery pack and starting from scratch. Probably worth running a mix-n-match pack until you really need to replace them as a unit.
You are saying that your battery charger is indicating "charged", but what voltage is it considering the job finished? Are you able to read the charge current going into the batteries during the charge cycle? As a rule, you can expect to return 20% more ampere-hours to a discharged battery than you took out. Is your charger failing to provide sufficient voltage and/or current to complete the absorb cycle of the charge? You can hammer 75-80% of the ampere-hours back in fairly fast, but the last 25% or so will be returned slowly as the chemistry of the battery changes less quickly as the electrolyte reaches the charged state.
Here's a copy-and-paste from an article I wrote for a New Zealand magazine several years ago:
Mr. Sharkey said:
Multiple stage charging is the best and most economical way to fill up batteries. Three stages are usually preferred, bulk, absorption, and float.
Bulk charging is done to return 75% of the energy removed from the battery quickly. Amperage is kept as high as the battery and charging source will allow, usually equal to a value of 10-20% of the batteries ampere-hour capacity ( a 220 ampere-hour battery would be charged at 22 to 44 amps, expressed as C10 or C5. The equation for determining this value is Capacity divided by charging current determines time to charge fully [C value] 220/5=44=C5) When the battery voltage reaches approximately 2.41 volts per cell (14.5 volts for a 12 volt battery), absorption stage is initiated.
In absorption stage, the current into the battery is limited to hold the terminal voltage at a pre-set value just below it's gassing voltage, usually 14.5-14.6 volts, for a specified period of time, from half an hour to several hours, depending on the specifications of the battery manufacturer. During this time, the battery is allowed to slowly absorb approximately 20% of the electricity being returned in the charging process. At the end of the given time, the float stage is initiated.
Float charging is a condition in which the battery is held at a specified voltage that is well below it's gassing point, but high enough that the remaining 5% of capacity can be returned. Typical float voltage for a 12 volt lead acid battery might be 13.1 volts. A battery can be left on float charge indefinitely without damage or excessive water loss.
It could be that your charger isn't set up to fully deliver the absorbition cycle. Either that or your hydrometer is whack or your battery chemistry is different than you anticipate and the specific gravity differs from the standard you're expecting.
About the only way to tell if the batteries are really fully charged is to run a full cycle load test, measuring the Ah removed with a calibrated accumulating instrument, then recharge, tallying the Ah returned.
You still have some detective work to do, something is giving you false indications, the charger, the hydrometer, or your batteries. If the cells are gassing, then they are pretty well full, so I'd tend to rule that one out.