simply Tag your 2DTk camera as "Main Camera" (its one of the default options on the Tag Unity editor
There must be always a main camera in the scene, properly tagged, even if its not the one in use.
This is an hold one, but myself tired off finding workarounds finally cracked it! An now Mouse Pick works
Putting it in in case anyone finds this via search.
PS: Additionally all the game window playmaker tips (wish I add forgot to exist) also need this "Main Camera"