In episode 225 of Semantic Mastery's weekly Hump Day Hangouts, one participant asked why some verified GMBs are not showing up in Google My Maps.
The exact question was:
Some of my service area business client's verified GMBs do not show up in Google My Maps, when searching for them in order to add their pin to a G MyMap. Why is that? Does adding a KML file to the MyMap rectify that?