Is it safe to put an enameled cast iron dutch oven in a pre-heated oven, when the dutch oven is starting at room temperature? Or will this cause thermal shock and break the enamel? The Internet says contradictory things. I have a recipe that says to do this (i.e. put it straight in the oven, without heating it first, with some uncooked vegetables and oil in it). This is the model I have, if it makes a difference: https://www.johnlewis.com/john-lewis-partners-cast-iron-shallow-casserole-31cm/p4055687 
The first thing to do is buy an oven thermometer to see what temperature your oven actually is. Technicians have no real way of knowing what the temperature is when you are using it; what you need is hard data. Oven thermometers are very cheap, and it will give you a better idea of what's going on. Put one in your oven and then turn it to the temperature you want, then see what temperature it actually gets to. Next, read your oven's manual thoroughly to make sure there's not something you're missing. One of my ovens wasn't working--I set the temperature and mode but it wouldn't heat. I thought it was broken before I figured out it wouldn't work if the clock's time wasn't set. Don't ask me why it was designed that way, it doesn't make sense but there it is. Some ovens have a start button and won't change the temperature unless you press it. 