[quote=@Joshua Tamashii]
really, you make it sound like a case of immunity. The heartless are growing resistant to the WDS.
[/quote]

Well they don't really know what it is. I mean, it isn't that (spoilers), but that's one thing it could be, which is why its wise to send in Keybladers to not only investigate the cause, but deal with any Heartless not affected by the WDS with the Keyblade. 

I mean, that is one of the things that has got King Mickey nervous, that whatever this strange not Heartless and not Nobody signature they picked up is could be just that.