Assigning access to folders in SAC

I recently moved models from the public folder in SAC and assigned rights to the now private folder, which created some conflicts with null values in some tables in the related story. I therefore moved the model back to the public folder but now no user could access the model. Previously any model in the public folder was available to all users. Was there a change in policy over this or did I do something wrong? Is this to do with the use of teams to assign rights rather than individual users?

The other issue with the null values I haven't been able to solve it without giving the end user the BI Creator role, which I would rather not do as this allows them a whole range of access when they are BI Viewer in reality.