Hi and thanks for using 2CarPros.
Chances are the ones you are trying to remove and can't are rusted in place. What I would suggest is getting a good penetrating oil and spray them down a couple times, waiting about ten minutes between doing it. Then, get a good 6 point socket and a breaker bar so you have extra leverage to get them loose.
Now, if there is a lot of resistance when they are coming off, loosen them a couple turns and then tighten the nut one turn. Do this back and forth until the nut is off. Also, spray them a bit when doing this. By doing it this way, you have less chance of breaking them because of dirt and rust
As far as not being able to get the new one on, confirm the threads on the stud are in good condition. Also, make sure the replacement nut is the correct size and thread pitch.
Let me know if this helps.
Joe
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Monday, February 11th, 2019 AT 5:56 PM