Gearmotor problems

[cosworth34677]

Ok I am killing the motors on my gearmotors on my dozer! It's more then strong enough but it seems that only the motor does! These motors are 370 brushed I`m getting these motors from Amazon they are 100rpm 12v worm gear 90. Motors. I just seem not to have any luck with them I just bought two Monday and now both are dead! The gear box is fine but the motor wont turn. I dont understand why they stop if you take them apart it looks fine . I dont know help! Any ideas to fix the problem I can't keep buying motors every other week !