supposing You have saved that assembly and opend new.
And further assuming that the parts are Inactive, so that they can not be used for further or more detailed information about the geometry.
Just use the Activate Part button and activate those parts which will be needed for measurment.
You also can see that parts are inactive by the color they have in pathfinder
orange = active
grey = inactive
this is an options setting for opening an assembly.
If those are small, medium or big You can define how to open them.
Activated, inactive, simplified, etc.
By definition parts are inactive for mdeium and big assemblies and that might be the reason!
Ahh thats great news! The active/inactive "issue" have been causing alot of frustrations for me.
Happy that I now know the reason and how to solve it.
Thanks a lot of the help