Answer: There are several possibilities. These plants tend to do well when fairly rootbound, so transplanting into a larger container with excessive soil can result in overly wet soil. Make sure you are not overwatering.
It is possible the roots have not yet moved out into the new soil. If this happens, sometimes watering the new soil leaves the old original soil -- where the roots are -- bone dry. Check the soils with your finger and see.
Another possibility is that they are now planted deeper than they were before or they have settled since you planted them. This can cause them to rot. If this is the case, take them out and repot higher.
As insurance, you may want to root a healthy piece and start a new plant.
I hope this helps you trouble shoot. I'm sorry about your aloe vera plants.
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