Then, put a light charge on it to top it off. Do that several times in a row and you now have a great big battery with nearly no usable power. The battery has been trained to have a shallow charge memory. Weird, but true. For best results a deep cycle battery should be worked hard , discharged heavily and then correctly recharged, fully.
Deep cycle charging is almost impossible to do correctly with a manual charger.
The battery wants to be charged rapidly at first and then have the balance of the charge load supplied at a successively slower rate. Correctly charged and maintained batteries do not deteriorate based on age. And, it will often happen from one day to the next. These facts bring about several questions. If not, something is amiss and you're not receiving fair value for your battery dollar and probably not enjoying the battery performance that you deserve.