First of all, we do not use our bettas for fighting here and deplore the practice of using the lovely fish for this barbaric habit. If you are using the fish to fight any number of things could be wrong and let me say that bettas are not as a rule "lazy" fish.
There would be a problem.
Do you have him in a warm (heated with decent adjustable heater); cycled and filtered tank of a size of at least 5 US gallons with no tank mates or restricted tankmates in a community tank. (not wise with a cambodian betta)
How much are you feeding and what are you feeding? Are you doing water testing and what types of testing equipment are you using? How often do you change the water and how much of the water do you change at a time?
Please read the following:
http://www.fishlore.com/Forum/32-bet...d-first.0.html
Rose