I would suggest you check your oven temperature. You might be baking at a higher temp than you think. Use a probe-type thermometer, the kind with the readout unit that is outside the oven. First, calibrate your thermometer. Boil a pot of water and place the probe in the briskly boiling water. It should read 212 F / 100C unless you live at a very high altitude such as Denver. If it doesn't, you can still use it - you just have to adjust. For example, if your thermometer reads 215 in the water, you know you have to add 3 degrees to the reading to get the actual temperature.
Now place the calibrated probe on your preferred baking surface in the cold oven and turn it on. Use a timer and write down the temp. every 2 minutes for at least 30 min. You need to know how your oven behaves. I find my oven will overheat by at least 20 degrees during preheating. Once it gets stable, it only varies by 5 degrees, but it runs too hot. I have to set it at 253 to get a 275 temperature. If temp fluctuations are a problem, try putting the probe inside a large lidded heavy aluminum pot inside the oven and checking the temp profile that way. that will give you much fewer fluctuations, so you just bake your pieces inside the pot. Some people bury their pieces in baking soda to bake. I haven't tried that. - Beth