T8 user, starting to get blade dishing by heel

[av8r]

I've been using my T8 with the supplied stone wheel for a few years.  This weekend I sharpened about 40 knives for family/friends and some ended up with some "dishing" (may not be the correct word) near the heel of the blade.  I check the true of my wheel before every session and don't apply heavy pressure (I think) when sharpening.  2 of the knives are my own kitchen knives and I've not noticed this happening before.  Any suggestions on what to look for or change in my technique?