llSetPrimitiveParams in Child Objects intermittent?

I am having mixed results when trying to modify the primitive params of a child object. Specifically, I am trying to mod hollow on a box. Sometimes it works, sometimes it doesn't. I also had this problem on another project when changing the cut of a child object.

Anybody seen this or have a work around?

Edit: I think I got it - modify perms are necessary even on script mods it seems. I think... but it is 5am on Monday (rubs eyes)

What sort of problems are you having?

I've tried stuff like this before. Certain operations like to crash the owner's client. Not fun.