|Image copyright REAPER BOOKS|
I once asked LEO BAXENDALE (via email) why there had been no further WILLY The KID books after the third one. He replied that the details were included in his book The BEANO ROOM & Other Places. Guess what? I acquired an autographed copy of the book yesterday, and although the topic is referred to, there isn't any specific reason given why the Willy books didn't continue, nor any explicit mention of Leo being informed that there would be no more of them.
(*Update: Unfortunately, Leo has passed away since this was first posted, so getting him to autograph something is no longer an option.)