You've followed the recipe (that might be the problem), and the dough is not right. A runny, soupy dough is almost impossible to work with. You might use the same recipe each time, but the results vary. If your starter is robust and healthy, then the problem lies elsewhere.
At least most the of time the problem can be attributed to temperature.
If it's too cold (under 60ºF) not much happens.
If it's too warm (over 75ºF) too much happens.
I have found that 65-70ºF is the best temperature for dough maintenance.
If somehow you can't your temperature down to the ideal, then consider a shorter proofing time and/or less sourdough starter.
Much too soupy