The battery could be the problem. Check the voltage of the battery across both terminals, then check it using the + side on the battery and the - side on a bolt or bare spot on the frame. They should be the same if you have a good ground to the frame from the battery.
Check all of the battery connections and confirm that you have a battery that will hold a charge. It should be over 12.7V before cranking and 10 to 11V during cranking. When running it should be over the voltage you got before cranking or there is a charging issue.