Hence the reason I sold my Neimi dies. Much easier to purchase them from a reputable bullet maker, with a known reputation.
Around 25 years ago, I went through exactly the same thing.
I had bought a complete 6mm bullet making set up from one of the reputable Die makers. I went through the usual learning curve, and finally got them to shoot pretty good, but could just as easily give Jeff Flower a call and get bullets that actually shot better.
I went through all of the trials and errors, and one day just stood up and said “to heck with it”. My presses and dies sat right there for darned near 2 decades gathering dust. I never even too them off the bench.
Then, several years ago, I decided to give it another try. I got a complete carbide set up from George Ulrich and made up my mind to make a competitive bullet.
I do pretty good with them.
What did I do different this time? Mostly attention to detail. I tried different combinations of core seat pressure, point up length, lube during core seat and lube during point up, taking good advice and coming up with a few ideas of my own, and lots of range time figuring out what worked best.
Also, I am lucky to have a partner. My Friend Ed Bernabeo, who is a few years older than I am and is retired, does the labor, (pulling handles). And, since we both shoot the identical 30BR combination, we have twice the input in how the bullets are doing. Ed has learned to look for any little anomalies during the actual handle pulling and will call my attention to it.
One thing I pay close attention to is the core seat punch diameter. I like it close. The drawback can be if you catch even a slight lead build up on the punch, cases will stick to the punch. The solution is to check it about every 20 bullets and keep it clean.