I used to have that problem. To determine where my problem was I took a flared and de primed case with no powder and adjusted my seating die until that case would fit into and come out of my barrel easily, then took bullet and placed it into a un primed case that had been flared but had no powder and started with the recommended COAL and slowly shortened it until the case would pass the plunk test as you call it. It all had to do with the ogive of the bullet I was using.
I have had that problem also is several of my handguns. I also found that the ogive of my selected bullet was the problem. and I also corrected that by playing with the OAL.