I can't seem to make my target nameplate scale when the target is casting, but it scales when non targets scale.

The casting scale value is overwritten by the target scale value. So, if you enabled a individual scale for the target, that takes priority.

There is no easy solution to this. You can disable target scaling - probably not something you want. I changed that some time ago because otherwise, the target scale is changed every time the target unit casts a spell. Also not great.