I only really know of one formula that primarily uses dextrin as a fuel. It's still unsourced, and honestly doesn't look like that great of a formula to begin with. It's at least out there, originally coming from Wouter Visser's webpage.
Potassium Chlorate: 60%
Sodium Bicarbonate: 20%
I don't really consider dextrin to be that good of a fuel. Sure it has some fuel value, but generally as long as it's used in relatively small amounts it can be added or removed without much change in the performance of the star. It definitely has less fuel value than red gum or other commonly used normal fuels. If you want to get a better idea of this look up some information from Mike Swisher or 50AE. They have quoted some real life values in the past for various fuels. Usually it will be in terms of grams of KClO3 to burn 1g of fuel, or the other way around.
Those sparks are definitely from coarse charcoal, and not anything inherent to the formula itself. Grinding it finer very well could change the performance and color. Are you sure the water bound stars are dry and not driven in? That's the only thing I can think of that would make them harder to light, and that much slower burning. Charcoal likes to hold onto water, which might be your issue.