I helps to understand what's happening. The boiling point of a sugar solution is determined by the percentage of sugar. The more sugar that's is actually in solution (not just in the pot), the higher the boiling point. If you remember the thing about boiling points, the solution can't get any hotter than its boiling point, no matter how much heat you apply or how long you boil it.
But here, we're changing the concentration of sugar by boiling off water. The soft ball stage is 235 to 240F. To be able to achieve that temperature, you have to cook off enough water to bring the concentration of sugar in solution up to 85%, and only when that concentration is reached will you be able to get the temperature that high.
Now, the other thing that's at work in this is that you can't dissolve that concentration of sugar in cold water. You could carefully measure water and sugar by weight so that the mix would be 85% sugar, but it wouldn't dissolve. But it will at 238F. You don't want to start off with 85% sugar, because as you heated it, you would cook off some water and drive the concentration (and the boiling point) too high.
So, you kind of sneak up on it by letting the thermometer tell you when you've boiled off enough water. You know, because you can then get the solution up to that temperature. Then, the science part happens. You remember that you have to have it up to 238F to make make an 85% solution, but now you're cooling it, and a cooler solution can't handle that sugar in normal solution. But if things work properly, the mix becomes supersaturated with sugar, and the sugar stays in solution.
This is a critical moment. If you stir it at this point, or if any particle of anything, like a speck of sugar, falls in, or you failed to brush down the sides of the pot above the mixture and left sugar crystals clinging, you get a particle floating free in the mixture, and the supersaturation of sugar will crystalize around it, and your fudge will be grainy. Same with scraping the bottom of the pot when you pour the mix out to cool. Don't do it. If there's a hardened chunk of something on the bottom, it will become the seed for crystals. Good fudge is what scientists call a supercooled liquid. A supercooled liquid is one that has cooled below the temperature where it becomes a solid, but without crystallizing, because we deprived it of a seed to crystallize around. That's why proper fudge is shiny like the surface of glass. Bad fudge is grainy from the sugar that fell out of solution or was excess that couldn't go into solution.
The bottom line on this is that you have to be patient. It just has to boil until enough water boils off. It can't fail, because the water will indeed begin boiling off after you pass 212F. (Actually, it will start a bit higher, because there's already some sugar in solution, making the boiling point a little higher.) If your recipe is a little light on sugar, it will take longer, and the temperature rise may seem agonizingly slow, but it WILL rise eventually. And better to start light than to start with an excess of sugar that won't go into solution until it's at a harder stage.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen