After the starters are put in, does the engine start?
How many amps is the starter pulling?
Well, to diagnose this problem properly, this would be the next step. There could be a few different things that is causing this problem.
ALso, lets do a voltage drop on both the positive and negative circuit when the starter is in use... (cranking).
Here is a very good article.
If the voltage drop test is fine and there is not high amps being pulled from the starter, the only thing it could be is simple a bad starter. I would have them warranty the starter rather than pay for a starter from Nissan.
The only else I can think of is if the motor is starting to lock and causing the starter motors to burn out. But then again, this will show up on a amperage draw test.
DId you have any further questions?
I hope this helps. You can always accept my answer now, and repost back again later on if you have any more questions. If you want to discuss it some more just hit "reply", it's free to do so otherwise don't forget to hit "accept" and leave positive feedback if you found my answer helpful.