Layout scale is locked

I’m tired, it’s the end of a long day so I’m sure it’s something obvious and stupid but can someone suggest reasons why my detail scale is greyed out?

Multiple details selected at once? Perhaps you have two right on top of each other.


Thanks for the reply.
Not sure how that happened, I didn’t have two viewports or anything else.
But the solution was obvious and right in front of my eyes - put a new detail in the layout and delete the malfunctioning one.