From proof to oven, rise lost

I posted on this thread; but I suspect it's too old :-)

I've been using Ken Forkish' Flour Water Salt Yeast - working my way through to biga using mostly KAFs and Bob's Red Mills.

The only thing I have not done is preheated my Dutch Oven - so far!

Although I now get a great rise, my proofed dough tends to collapse when I invert the banneton and place the dough - very carefully - into my Dutch Oven.

Is it that:

1. I need to be more dextrous/careful; or
2. the structural strength still is innately poor - I'm still experimenting with the length and frequency of folding?

Thanks for any advice!