I have a Mec 9000G and have messed up a number of rounds. Just like posted earlier Machines have no will of the own, in other words Machines don't make mistakes-owners/operators do!
Back to metallic cartridges, I've owned a Dillon since 1984. It started out as a 450 and has been upgraded to 550 B status, with a case feeder. Starting with the case feeder; It will make loading easier, but it won't make loading better or faster. I would not buy it again. It was a bear to adjust and will jam on a short stroke-which happens when you stop to keep from making a bad round.
Now about bad ammo. I never failed to prime a round, until I installed the case feeder. This is astounding considering that the 450 had a manual priming bar. Also astounding is I never failed to drop powder either. The 450 had a push bar powder drop (and it was astoundingly consistent with ball powder). Things to avoid are, #1-TV, #2- wife or children, #3- well meaning or curious friends. Music in the background covers noises that might distract you. Every hour or so you should take a short break, get a drink etc., check in with the family (so they won't come looking for you) and reload your primer tubes. This is also a great time to line up all the components for the next hour (think in terms of 500 or 600 Rounds. If the wife says 1/2 hour until dinner, only set out 300 components and let her know you'll be there when your batch is done (follow thru on this and they leave you alone when needed!) Back in the early 450 days, I loaded 1 batch of 223. It was 20,000 rounds it took weeks of loading, and I have about 6000 left. They have been flawless, and In a bolt gun shoot less than 1/4" groups @ 100 yards. After that I loaded 9mm in 10,000 round batches to feed my full auto MAC-10.(I did 4 of the batched without any bad ammo produced. Now days mostly do a years worth of ammo per cartridge in a single batch (45 ACP is about 6000, 45 Colt is about 4500 and others in like quantities).
Get a system, pay attention, and you'll make good ammo. Have fun. Ivan