If I want to help Johnny, kids would become a behaviour issue.

How do you deal with it now?  When I taught the traditional way I still had to deal with this problem since I couldn't be everywhere.  I am not going to tell you this is the magic method that solves all problems.  Problems you have had in the past may or may not exist in the Flipped Classroom.  This issue was a problem for me before and remains an ongoing problem.  Sorry to disappoint.