RE: Using Turkey Necks in Stock
A few things. First, don't give up. Stock making takes some practice and before you know it, it will be a breeze to quickly put a stock together.
Were the necks stuck all together in one big frozen bunch? If they were, it'll take longer to fully blanch the bones. Make sure to start the stock in a clean pot. The gunk on the bottom of the blanching pot is nasty so you'll want to either transfer the bones to a new stock pot (if you have a second one) or place the bones into a large colander while you wash out the pot.
Perhaps the heat was a bit too low. In order to prevent the impurities from emulsifying with the liquid, we want to drive home the point that stock should never boil; however, it can simmer gently. The bones won't totally break down during the stock making process. You can chop the necks into smaller pieces (3-4 inches or so). The stock will be more concentrated because you'll need less water to cover. Finally, for the mirepoix, you don't want them too small. One-inch pieces are fine as you don't want them to disintegrate and cloud the stock during the long cooking time.
You can even make a small batch with a leftover carcass from a roast chicken. We often make tiny batches because they are easier to handle. Stock is one of the most valuable things you can have in the kitchen and it will take your cooking to a whole new level, so keep trying. Hope this helps!