Aquatic plants are a special breed, crytps while great and can last you a super long time can be a bit finicky.
reasons for plant melting (not just crypts)
a-they were grown out of water and you put them totally into water. in this case they will drop their leaves and regrow ones suited to grow in water
b-new root location. same thing as above the will regrow
c- water params not right. in this case they will look different then the "melt" look, and can have holes and yellow spots on them
what you have sounds just like normal melt, just leave them be and make sure they get good light and some ferts and liquid cow (api co2 booster is okay but i love flourish excel, seachem brand!) and they should be fine! a planted tank takes about a month to feel really filled in :)