I'll try to explain the problem in most simplistic way :
Problem
-- Batteries are supposed to be recharged at slower charging rate like 10a/h meaning a battery of 100 AH should ideally be charged in 10hrs. This method is called "slow or trickle charging".
-- Now the charging system in UPS is designed to fully charge a battery in 3-4 hrs as the requirement asks for it. Now you are charging at a higher rate, resulting in over-heating of battery this consequently effects the specific gravity of acid used and slowly damages the plates of battery (don't ask how just believe it).
-- Overall a battery used for UPS has shortened life and requires a slow charge cycle every now and then.
Solution
-- There is no solution for reduced battery life when used with UPS, we all are facing it. If the solution lies in use of dry batteries or something like that then I'm not aware of that.
-- For your present problem take your battery to a battery shop get its specific gravity checked and adjusted ( i think it is 1240 in summers). get your battery charged at the shop, it might take a day. And get the cells checked for boiling like symptoms. If 2 or more cells in a 6 cell battery are boiling then it needs replacement