Can't change equations with 'edit all' option

In an assembly I've created some equations, no problem sofar.
Before i assign them i can edit them all with the 'edit all'option, very comfortable.
When i asign them to parts within my assembly i can't change therevalues in the 'edit all' window.
SW says the part is read only but as far i know it isn't.
What am i doing wrong ?
SolidworksAssemblies