I've found that those who legitimately have a clean record don't give up fighting a bad NICS response. That being said, I've known quite a few Denied that thought that those youthful indiscretions didn't count after a few decades. When I say that I've known them, they were customers when I worked in a gun store, not personally.
I had just turned 16 the month prior to breaking into a gas station and stealing cigarettes. I plead guilty as a youthful offender and was given 5 weekends in County Jail. My record was supposed to have been expunged when I turned 18. I joined the Navy at 17. I asked the recruiter if I needed to mention it. He said no, so I didn't. I had been in Boot Camp a few weeks when I was called to JAG and asked why I neglected to tell this. I told them what my recruiter told me and I was allowed to stay in. Fast forward to last year. I applied for a Suppressor and was denied after 13 months. I requested my FBI file and sure enough there it was. I had a hearing Friday and my petition to expunge and seal that record was granted. I told the judge all I wanted was a clean FBI file and he told me that I would be given that with Friday's ruling. This all started 52 years ago.
I have only bought 2 guns from an FFL. Both of these were after I had gotten my CHL (I was also asked about my juvenile record but approved) and did not go through an NCIS check due to the CHL. It amazed me that something so long ago would still cause so much problems in today's world.