What almost nobody mentions is that the water ratio is the first thing: with fresh vegetables you need way less liquid than it seems logical. I use roughly one part water to two parts vegetables by volume, not the other way around. Also, cooking uncovered for the first 10-15 minutes helps moisture evaporate and flavors concentrate, instead of everything just floating around in bland broth.
The other important thing is that some vegetables release way more juice than others. Zucchini, eggplant, and tomatoes are water bombs, so with those you gotta be generous with salt and spices so the flavor doesn't get diluted. I always add a strong base sofrito (onion, garlic, carrot sautéed in oil) before throwing in the rest, that gives a background flavor that doesn't depend on the vegetables doing it.
And one thing: if you already messed it up and it turned out too watery, at least you can save it by letting it simmer uncovered for a bit longer so the liquid reduces. It's not ideal but it works.