Hi Brian,
This is one for the knowledge books if it gets figured out. Only possibility that I can easily come up with is which way the weave was running when you attached the silk like Larry asked.
Personally, I never had a silk covering job loosen after all that and humidity, etc., around here can be all types of extremes. I will add that I use more "shrinking" dope on silk than on silkspan, probably double the amount. I never use over 4 coats of high shrink on the open bays of silkspan but I will use 6-8 on silk. I use everything thinned 50/50 so it cut that in half if you are using full strength. Mainly because it takes more material to fill silk, and I just got in the habit of using high shrink to fill it. Plus, silkspan pulls tighter as it drys than silk. I do pull the covering material as tight as I can while putting it on, and keep both sides wet while covering. Then put a coat of dope on the opposite side as quickly as possible when filling it with the high shrink.
One thought is to use a heat gun like when using polyspan. Do one bay on the bottom then one on the top, working out from the center. This might help to cure the dope and tighten up the silk. Low temp and work quickly to not cause any charring, etc..
Hope the problem gets solved!
Big Bear