How long have you been using it? I assume you don't have a meter?
The light might still work when the battery can't hold a charge anymore.
To be sure, you really need a meter to measure the voltage at the connector.
Is your charger working ok on your other
batteries?
Have you tried it with a different cartomizer?
Does that carto work with other batteries?
I know it sounds obvious, but your carto could be fried and if you only have one, you might not be able to tell and think its the battery.
Those batteries don't last forever, so it could simply be worn out.
Possibly the battery post needs to be pulled up on the Riva.
You need at least a cheap multimeter though, for problems like these. No vaper should be without one.