This is a mystery! You pulled the wet silk tight. It was tight when dry. Brodak clear?
The first couple coats might look awful! ---Until about 1 hour, when it usually (always) tightens up again. After a couple of brushed coats, I begin spraying clear. It doesn't get loose after the first couple coats.
Silk expands when wet. So that even when dried out, you can still wet it again and it will expand the same amount. So it isn't a matter of a "one time shrink" situation.
It isn't necessary to pull the wet silk really tight, but it does make a nicer job of it.
(Just thought: I always give the wood frame 3 or 4 coats of clear so that the silk will stick well).
F.C.
I've used silk from Thai Silks in (Los Altos, CA) with good results.