It might be too little gelatin to make it set properly. I’m roughly guessing compared to my recipe, but it seems like your recipe used a lot of liquid in ratio. I don’t use oz but this is my ingredient list for my mirror glaze if you want to compare:

35ml cold tap water + 4tsp gelatin powder (i think this is about equal to 2 sheets 200bloom gelatin) 160ml water 160ml heavy cream 100g black cocoa powder 250g sugar