I had the same trimmer. I used a few pieces of new brass to set it when I started. One thing is not to push in too hard when turning the brass. Another is make sure you have the trimmer mounted to something solid. I tried at forst to work with it on a table, not pushing in on it too hard. Lengths were all over the place. I mounted it to my bech with good nuts and bolts very solidly. That helped out a great deal.
I traded it off for a lead melting pot, after I got a Lee trimmer lock stud, and case length guage for the caliber I was trimming, and it works with my drill.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.