"I stored it in my basement over the winter. I charged it up last month "
There you go, you killed it. As wet cells discharge, insoluble sulphur compounds form and the battery takes permanent damage in just 30 days. Every day beyond that--and the battery is turning into a brick. Sometimes, you can get away with ignoring it for six months and still have enough capacity to get by for another season, but not reliably.
Write if off as a battery lesson, and replace it with an "AGM" deep cycle battery. AGM batteries are built differently, and they can withstand six months of sitting in the corner without any permanent damage. Although, I'd still put a charge on it every month, or every other month, because "can" doesn't mean that's best for them. Since they don't self-discharge much, you won't need to charge them up for very long, either.
The Optima spiral-cell batteries are the big brand name in AGM batteries, but you can save a good 30% off their price by buying a flat-plate AGM battery from any distributor, many companies make them these days.