Why does melted chocolate that is simply cooled back down often fail to regain a shiny professional finish?

Answer: Cooling forms mixed crystals

Cocoa butter separates out — This is close to fat bloom, so it is a plausible guess. But the question is about simply cooling melted chocolate back down, where the cited issue is mixed cocoa-butter crystal growth rather than fat visibly separating first. The useful distinction is surface bloom versus a poorly organized set.

Cooling forms mixed crystals ✓ — Correct. Simply melting to about 40-45 C and cooling to working temperature does not reliably produce gloss. Natural cooling can produce a mixture of crystal forms instead of a Form V-rich network. The hidden lesson is that the cooling path, not just the final temperature, matters.

Sugar grains become larger — This points at sugar texture, which is not the main tempering failure described here. Sugar bloom involves moisture dissolving and recrystallizing sugar, but simple cooling trouble is about cocoa-butter polymorph competition. The contrast separates humidity damage from fat-crystal control.