what does flip flop in visa means? how many times consider flip flop?

I am MD and I entered USA with J1 research visa,self funded,then matched into residency program,my prelim program support H visa and my advance program offer J1 clinical visa,after 2 year J1 research ,last year I changed my status to H! and now I want to change it to J1 clinical,but ECFMG does not support it,saying although there is no rule but since it is flip flop in visa, we are advised not to help you!