The chorizo you find at the grocery stores is of the Mexican variety, high in secondary meats, added fat and tallow as well as organs. The Spanish or Portuguese variety is heavier on the lean cuts and much less fat and organ meat, as well as having a longer smoking and curing time, which makes it shelf-stable. The grocery store variety must be refrigerated.
I would guess that if you want yours to soften and melt more, you have to add more fat and organ meats. Now, that's my thoughts- I've not made sausage so you should ask the pro's for concrete knowledge, such as Pops.