If you've use Din's Fire once on the current screen, on subsequent times the Up Rocket Sparkles don't appear.
I'm not sure why this happens... examine the calls to addsparkle2() in link.cpp.
Using Din's Fire twice per screen
Only one weapon of each phantom type can sparkle on the screen at the same time, and there was no code to ever delete the rising Din rocket... it just kept going up, up, up off of the screen, and hogged the sparkles.
Fixed in the next build.