4 month old batteries won't stay charged long

I recently installed 6-6 volt US batteries in my Ez go about 4-6 months ago. After installation, the batteries would last for 10+ miles slite grade. As of now, I'm lucky to get 2 miles before they are near dead. What happened here????
(When I plug cart in, it charges for about half the cycle time. Batteries show full charge. I have 2 chargers and it does it with both). Thanks for the helpin advance!!!!!!