Kneading in some glycerin will help, they sell it at Michael's. Or you could knead in more shortening. Careful not to add too much or it will get too stretchy.
It could also be that you overcooked your marshmallows on this batch and crystallized the sugar in them. If that's the case, when you re-microwave, you will notice that it seizes up again very quickly while trying to work with it. Unfortunately, there is no really good way to fix it at that point and you will have to make another batch.
Good luck!
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