Compare the case head to shoulder datum length of the once fired, re-sized brass to that of the new Lapua brass. If the brass once fired in somebody else's 284 Win. chamber and full length re-sized per the setup instructions of most manufacturers' dies is longer from its head to shoulder datum than is the new Lapua brass what also chambered fine, then your chamber from bolt face to the chamber's shoulder is shorter than what your shellholder and 6.5/.284 Win. FL die combination is capable of producing, assuming there was maximum insertion of the case into the FL die with it adjusted to contact the shellholder plus another fraction of a turn, as per the typical manufacturer's instruction.
If not, adjust the die down to contact the shellholder plus just enough more of a fraction of a turn to cause the press to cam-over with full extension of the press ram. If the case still ends up too long from head to shoulder datum, then load up and shoot some of that new Lapua brass, re-size it, and try 'em for fit in your chamber.
If after firing and resizing the Lapua brass, it's also too long from head to shoulder datum, then either take off what you need from the topside of an inexpensive shellholder, or from the more expensive and hardened bottom surface of the die. "What you need" is at least the difference in head to shoulder datum length of the old brass vs. new, unfired Lapua plus another couple or three thousandths more as insurance.
If the problem has nutin to do with headspace, then you gotta find where there IS interference to chambering.
