Hello there! I would be happy to help you troubleshoot your aspic issues for your upcoming outdoor event. Aspic fails to set primarily because of an improper ratio of collagen-rich connective tissues to liquid, or because it was simmered for too little time to fully extract the gelatin.
To achieve a natural set without additives, a reliable meat-to-water ratio is typically about 1 pound of high-collagen parts (like pigs feet, knuckles, or necks) to every 2 pounds of lean meat, covered with just enough water to submerge everything by an inch or two. You need to simmer this gently on low heat for 4 to 6 hours. A rapid boil will emulsify the fats and muddy the broth, but a long, slow simmer breaks down the collagen into gelatin effectively.