Oh then it is too soon to need a new battery. You could be overloading it - can you send all the devices you have connected to it ? Components of PC, and other devices. You would know approximate power consumption of those devices, or others here might help. If you are overloading it, you may face such problems.
If load is high, and you cannot upgrade your UPS, keep only important stuff connected to UPS battery backup port. Connect sensitive electronics like monitor, speaker etc. to surge protection port of UPS. And connect other stuff like chargers etc. directly to mains bypassing UPS.