It’s not hot enough when the oven “dings” !!
Many ovens “ding” when the air temperature reaches the specified temperature. You open the door, all the air rushes out, and the heat is gone. That doesn’t work for bread. Bread requires a very hot oven. The oven has to hold it’s heat for the few seconds when you open the door to put the bread into the oven!
So how do you know when the oven is hot enough? Flick a few drops of water against the side of the oven. If it vaporizes (turns to steam) instantly the oven’s ready. If you’re not comfortable “flicking” water, invest in a cheap plastic spray bottle that you will only use for water, and spray the side of the oven with water. If it instantly turns to steam and doesn’t run down the side your oven is hot.
Flicking or spraying the side of an oven is awkward, so note the time you start the oven, and the time when the oven is truly hot. After a few times of doing this you’ll know how long it takes your oven to reach the proper heat. (My gas oven takes half an hour if I’m not using a baking stone.)