SLA batteries should to be charged straight after each use, they do not have a memory. They also don't like being drained too low before charging, and should not sit for long periods without charging.
The fault may also be the charger, not the battery. I use a Ctec - 6 stage battery charger, and leave it on charge 24/7.
A good quality battery charger is essential, to preserve battery life. I learnt the hard way, before buying a good quality charger.
2015 Adventure Island. 'Ghost Rider'