Global Scale Factors - Unable to change

I have previously been able to change the Global Scale Factors (both within the DGN file I am currently using and others). Within one particular DGN I am now unable to amend it. I need to set the scale before placing my profile and text. 

Note I have tried using the function through Power InRoads Tools menu drop-down (via Application Add-Ins) and through the Project Options-Factors tab and unable to change the scale in both. 

Why might this be happening?

Many thanks.

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