SLA don't usually need balancing.
But they need a certain charging regimen, a 3-stage charging. This is the only thing that can be said with all certainty at the moment. Without further data it is all guessing.
4*12V doesn't tell me anything. Four batts can be wired either in parallel or series, or series-parallel. The result will be output either 12V (unlikely), or 48V, or 24V.
For 12V battery the charger should FIRST bring it to ~14.5V, THEN keep it at this voltage for 30-90 minutes, THEN drop the voltage to ~13.6V and keep it there for at least a few hours. This is how proper 3-stage charger should work. Most automotive chargers are not 3-stage. Yours - don't know.
In any event, measure the voltage on open contacts before charging and 10-12 hours after charging. It sounds like your battery is not charged. Possible reasons: bad battery, bad wiring between those 12V units, bad charger. The chart for State of Charge vs open circuit voltage:
For 24V - double the above numbers, for 48V - quadruple them. The chart is for regular "wet" battery. Sealed AGM batts have slightly higher voltages. Also, there exist hybrids of AGM and gel, with different voltages and different charging regimen. Buying a no-name SLA from unknown Ebay vendor is probably a bad idea.