@Piggy, the short answer to your question is, that I don't know.
I have used Frisbee's fondant recipe and until this month, I have have variable mileage from it, but as Rosti has pointed out to you, the problem was in the sugar boiling stage.
Let's look at some facts. the Frisbee recipe calls for 1/2 pint water to 2lb (1kg) of sugar and a table spoon of liquid glucose (buy a big bucket, not a tube from the local supermarket). Be generous with the glucose. I used a desert spoon and as much as would make the trip from bucket to put.
Like you, I turned up the wick under a stainless steel soup pot, with both a 2kg and 3kg (sugar weight) batch on the lowest light that I could manage. My thinking was as follows:
If I make up 4.5:1 syrup, then I know that when it cools, that the supersaturated syrup will inevitably crystalise. I used a flat wooden spatula as recommended and ensured that the sugar was all dissolved before it started to give indications of coming to the boil.
If you make jam, you know that once you start to boil the sugar, then the friut will no longer soften. Similarly when you bring sugar to the boil, any crystals that haven't dissolved are unlikely to, so take it slowly.
Once you have your supersaturated sugar syrup in the sink and start stirring, you only need to remember that it can't do anything other than crystalise as it cools. Stir with your spatula, taking any thickening components from the sides and bottom of the pan as you can and stir in. Eventuall it will start going milky and personally I like to start dispensing it while it is still very runny, as by the time I am finishing, it is stiffening up both in the bags that I use and in the pot.
I suspect that with Ambrosia, bringing it to the boil is likely to make you toffee and is at best an expensive way to make fondant.
I am happy now that I can get consistently repeatable results. The difference between the 2kg and 3kg sugar weight batches, was that it took a significantly long time stirring before the 3kg batch gave indications of becoming milky, but all you have to remember is that sugar can't defy the laws of physics and will crystalise.