I found the following graphic over on Fr. Frog's web page under
Developing An Accurate Load. Could this be what is causing the problem?

He goes on to say, "Because bullet tips vary slightly in shape seating a bullet by means of its tip leads to varying seating depths. It may be necessary to alter the seating plug by drilling it out slightly so it bears only on the ogive. Some manufacturers will custom cut seating plugs for you."
Also, some reloaders are using other techniques to match their seating plugs to their favorite bullets. Here's a link to a thread discussing the use of
JB Weld and other potential solutions...