I keep several windows open at the same time and a little button appears for each one in my taskbar. Why do some disappear as I add more?
They didn’t really go anywhere. When the task bar fills up it scrolls up to make room for more buttons.
You’ll notice that arrows will appear next to the open window area of the taskbar. Click up and the older windows appear as buttons.
If you want to avoid this, you could always make your desk bar a double decker. Just put your cursor at the top of the taskbar so it turns into a double arrow. Hold down the left mouse button and drag up. This will make your TB higher allowing you to see all open window buttons.
This takes up more desktop space, but at least you can see everything that’s open–at least until you open up even MORE windows