@hollie I use some J Herbin shimmer inks in pens with larger nibs (stub, broad) including Émeraude de Chivor. They work. I would be less confident in them with an EF for sure. Things that might help: make sure you're doing some things to gently agitate the ink while writing (when you pause, rotate the pen in your hand, flip it around, etc). Also, when not in use, store nib up.
@johns Thanks for the tips! I'll do that! :)