why can't I craft "+ fire damage/ignite" on an item with +1 fire, +28 fire multi?

Hello,
I found a synthesized Opal Sceptre with "of the ardent" +28% fire damage multiplier on it in gold. This seemed perfect for my Righteous Fire build.
I scoured it, transmuted it and spammed alts and augs until I got "+1 level of all fire spell skill gems"
However, when I went to craft on some damage, I can't craft the prefix "increased fire damage/chance to ignite" I get "this item already has a mod of this type."
All I've got on the scepter is:
implicit 40% elemental damage
prefix +1 level of all fire spell skill gems
prefix 19% spell damage (from augmenting)
suffix: +28% fire damage multiplier

I don't get why I can't craft the prefix "increased fire damage/chance to ignite" I've seen other scepters with both fire multi and increased fire damage. Does the +1 fire gems block it? Does it have to do with the synthesized status?

the best I could do with it was craft the prefix "28% increased damage over time" which leaves it short of my existing sceptres in terms of damage. Disappointing.

Ok, thanks