Warm it in the microwave and knead it. This sometimes helps.
You can try adding a tbsp of glucose and 1 tbsp of glycerine to each batch. Warm up the batch in the microwave (maybe 20 sec) and then knead it in. I always add these in when I make a batch of MMF.
You can melt some more marshmallows and knead those into the warmed up fondant.
You can add 1 tsp to start of tylose. Knead it in and let sit a few hours.
I have had great results in restoring MMF to pliability by using a bit of glucose and knead for several minutes and then add a bit more. The amount to add varies according to how dry the MMF is and how big an amount of fondant there is that you are trying to condition. Occasionally I also have to add in a bit of water (just a few drops). This may make the fondant a bit sticky at first but just keep your hands with a light sheen of shortening and keep kneading. If necessary then alternate back to a bit of glucose and knead some more. This always works for me.
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