Why does my sourdough bread always fall when I bake it. Drops by about a third; any Ideas as to how to prevent this? Many thanks, Reed

Why don't you try patting (lightly) yeast on the bread. Maybe that would work :)

TY It took almost a year for an answer! Happy New Years . Reed

Your welcome

To me the answer has to do with the gluten content of the flour you are using. If the gluten content is too low then the deflation happens. Otherwise, it could be that the gluten content is OK, but the proportion of sour dough starter you use to the flour is too high. I would imagine that the gluten in the sourdough starter has weakened already and so if you add too much starter, then the dough is too weak.
Think of it this way. The bubbles that keep the dough raising will eventually pop, just like the bubble gum you blow. So if the gluten is either not developed or is not enough, it will burst prematurely. Like when you first start chewing on the bubblegum, before the sugar dissolves, you cannot blow a big bubble... Likewise if the gluten in the dough is not developed enough (either by mechanical or chemical action) then the bubbles will burst prematurely.
Does this make sense?