Thanks for all the helpful comments.  I think I may be guilty of ignoring Lee's instructions and operating my press without a case inside the die.  Ever since reading John Alexander's (I think) comments in The Fouling Shot on how to operate the press/die combination I have followed his advice--which is to skip Lee's instructions to "use 25 lbs. of force at the bottom of the stroke and (optionally) turn the case 180 deg. and repeat."  John said it is only necessary to use a light touch--just so you feel the "cam-over."  I've done this ever since.  On closely examining my die I cannot see that I have caused any damage, but I carefully cleaned and polished all the parts--hopefully that's all it will take.