Larger molds can crack more easily if you move them before they are fully dried. I usually leave them in place for at least one full day. I do flip most molds right side up and remove the clay as soon as the plaster sets in less than an hour but larger molds (16 inches or more) can be trickier. If you leave the large mold as is (do not flip) for 24 hours, the clay will not dry out since the plaster will keep it pliable.
If the mold cracks and the breaks are clean (2-3 sections), you may be able to repair it. Remove the clay and allow the mold to dry in place for several days to a week. Next, clean the clay residue from the inside of the mold using a damp towel. Take each mold section and place it in the kiln and reassemble your mold. If you have high temperature wire, you can wrap it around the outer edge of the mold to secure the sections from moving.
Next, you can mix up a few tablespoons of the plaster/silica mix and smooth it on (or pour gently) over the cracks. This mixture dries instantaneously so you have to work quickly. In an hour, you can then gently sandpaper any bumps or unevenness on the repaired areas.
Finally, you can then place your glass in the mold but should monitor it when it the glass reaches melting temperatures in case the mold does not hold and allows spillage of the glass. I have used this technique a few times but it is not guaranteed.
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