Besides a battery or few batteries (load test them, to be sure they are OK, don't assume because you try more than 1 battery they are both OK)
sometimes "tip bounce" can happen from some loose wire somewhere, or a bad driver module, weak springs or worn out insulation and guides in the contactor assembly, which can also cause tip burning. when you replace the tips, have you replaced the entire contactor assembly or are you just changing the tips. if you replace the contactor assembly, are you getting the correct amp load (I see they are 300amp, what is the fuses?) contactors?