Fudge didn’t set after doing it twice

Like so many things, if you know what you are trying to do, fudge is not difficult to get right.

Fudge is a high fat candy. In the US chocolate is implied but the addition of chocolate doesn't change the process.

You want a solid, creamy candy with a smooth texture.

As Elendil wrote, it is important to ensure that you cook the candy to the correct temperature- but that is only half of the problem.

As the candy cools it will form crystals. In order to have a smooth texture you have to make those crystals as small as possible. The bigger they are the grainier the candy will be. The slower the crystals form the larger they will be able to grow.

For a smooth texture we want to form crystals as quickly as possible.

Crystals require a trigger to set them off- such as a seed crystal or agitation. If the candy is allowed to cool undisturbed and with no seed sugar crystals in the pot no crystals will form. It will be like a bomb ready to go off at the slightest provocation- this is what we want. After it is cooled we stir it like mad to form all our crystals all at once- fast and small.

Fudge is very forgiving in that it is easy to start over. Unlike an egg custard that when it breaks is unrecoverable- the sugar crystals that ruin fudge are easy to fix. You just add a little water, melt the candy back down, and start over.

Tricks to prevent premature crystallization

• Prevent errant crystals!
  A lid is placed on the pot for the last few minutes of cooking. This causes condensation to wash any errant sugar crystals off of the sides of the pot. A single errant crystal can ruin your fudge.
• Don't agitate the candy
  When the candy is cooling it can't be stirred or bumped.
• Add distractions
  I consider this cheating and it is unnecessary but some recipes reduce the risk of premature crystallization by adding things like corn syrup or marshmallow creme.

I like Kristina's suggestion that you research some of the basics of candy making. I'll answer those questions only briefly.

Generic Candy Questions

• You don't need to heat particularly slowly
  This is not like an egg custard where the speed of heating will affect the curdling temperature. You're just trying to get water out. Just don't heat it so fast that you burn the bottom or overshoot the target temp.
• Temperature range
  The different candy stages have about a 10F range. 2-5 degrees isn't going to make much of a difference but 7-12 will start to.
• Adding volatile flavorings to candy
  Many flavorings are lost to heat. Such flavorings are added at the end because if they boiled the entire time they would be gone.

Fudgy questions

• Stirring
  Stirring isn't required. The syrup won't burn unless you have the heat too high and are using a very cheap pot. Additionally, the syrup absolutely should not be stirred in the final moments of boiling because a sugar crystal might fall in that doesn't get dissolved. During cooling stirring would destroy the fudge because it would all crystallize.
• Drying out
  This is not a problem. The fudge will not dry out in the time it takes to cool.

• Losing the gloss
  As I wrote in the other answer, the syrup is concentrated and then cooled to a super saturated state and not allowed to crystallize. When it has cooled sufficiently it is mixed like crazy to form your crystals all at once. When this happens the shiny, glossy syrup suddenly fills with crystals and becomes thick and loses the gloss.

  It is important that the syrup be cooled enough that it is ready to crystallize at the drop of a hat. Again- if you stir it early, as it cools, you will end up with rock candy (more or less). I just leave my candy thermometer clipped on until the target temperature is reached. Make sure the thermometer is clean- one stray sugar crystal can ruin the batch.