Melting plants after propagation?

This has happened a few times to me now. I'll get excellent growth on some of my plants (wisteria, corkscrew val, etc.) and they start propagating like crazy. They'll send out runners and shoots in all directions. When I finally decided the propagations are big enough with adequate roots to cut off and replant, they all start to melt and die off once I have done so. Even the mother plant will wilt and then stop growing/sending out runners. I cannot for the life of me figure out why. My tank conditions were obviously good enough at one point to get good growth. Any ideas?