While unloading some unfired rounds from this rifle, one of them has the bullet pushed in so it's almost flush with case mouth.
If you still have it, measure the case length of that one round where the bullet pushed back. My guess is that it is slightly shorter than the rest, and that was enough different to turn your "slight crimp" into "no crimp", with predictable results.
UNIFORM case length is important to uniform crimps. When you set your die, cases longer than what you used to set your die get overcrimped and cases shorter don't get enough crimp.
My method is to use a factory round. run the ram all the way up, with the seater die backed out (and the seating stem backed out a bit in the die body, so it won't contact anything.
Screw the die body down over the factory round until the crimp shoulder of the die body
firmly contacts the loaded round. (hand tight). set your lock ring there. Adjust the seating stem down to contact the bullet nose.
remove the loaded factory round and you're ready to seat and crimp in one step. Minor adjustments may be needed, your first few rounds loaded will show if this needs doing.. Cases must be uniform length, and if you change bullets, you'll have probably have to adjust seating some.
I use Lyman and RCBS dies, the seater die removes case mouth flare, THEN crimps (when adjusted to do so).
Been using this method for 50 years, it works for me.