The type of battery influences DOD lifetime cycles. Here is a brief description
about what to expect.
Summarizing the article: with 50% DOD you get between 1-3 years of lifetime out of your batteries assuming you charge them up to 100% and discharge to 50% every day.
If that is the case I recommend you get a much bigger battery bank.
On the other hand you might not measure right. You need to buy a 'fuel gauge' type of SOC meter that is doing 'coulomb counting'.
As an alternative you can disconnect ALL batteries and let them sit for 24h and measure the voltage of each battery. That will also tell you if one of the batteries is bad.
As a rule of thumb: a battery healthy is full when the absorption rate is between 1-3% of its capacity. Lets say you have a 55AH battery then you would see the current drop to 0.5 - 1.5A at 28.8V.