I think Growerofthemonth is right. Looks like they may have been getting a bit too much water. Let them dry out a bit then resume feeding and watering lightly, give the roots time to grow into the substrate. If it is too wet the roots and beneficial microbes can die off.
What you need to do is not worry. Looks like it was started before the transplant. My thoughts are it was causes by over watering. You could see how wet the medium was at the bottom of the transplant.