Don't close the vent. The chicks will die at the end because they need ventilation.
The temperature likely needs to be increased--get a better thermometer. Brinsea sells a very good one for about $20.00.
Humidity goes down as temperature goes up. Humidity is base on surface size of water and not depth so fill a tank with a small surface area first. Try to get humidity to 35% for the first hatch and see how the air cells develop.
I have a brinsea too. I dont add any water until the 18 day ( I live in California, not much Humidityt) This last hatch 18 out of 24 eggs. the last 6 eggs weren't fertile. So 100% hatch. usually at lest 80 %. I set them, I don't touch again until day 18, fill one channel with water. Oh vent is open. Can raise humidity by particle closing vent.
My brinsea is almost 3 years old have had good hatches with it. It is getting tired too.
PS I would like to join the hatch along my hens laid me enough eggs.