You could try to take the battery cables off the battery and clean the battery post real good with a wire brush, clean the cable ends real good also.
Could be you are not getting the full voltage to the battery. Also make sure the cable is tight on the starter
Check the voltage in the battery, maybe charge it up. You might check the water/acid level in the battery, if low only add distilled water and not tap water.

I'm not saying that is the problem, but is real easy process to try and could prove to be a cheap fix.