Lost connections between Blocks when re-opening the Model

I have blocks in which i have enabled "modify its contents" under the tab "Initialization" of the Mask. When ever I re-open the Model, the line connections between the Blocks is always broken and I have to re-connect them all over again. I have tried

set_param(gcb,'Lock', 'on')

but it does not work as its a Model and no Parameter named 'Lock' is there in the Mask.