I’ve never experienced this problem with any of the Mayfly boards on any of the various PCs I’ve used them on, but I do have a colleague who has a similar problem with another CP2102-based board. I think his solution was that sometimes it takes plugging and unplugging the device a few times before Windows finally recognizes it and assigns a com port. Various people report similar issues on SiliconLabs support forums, and there’s never been a consensus on what the best solution is. Make sure you’re using the same high-quality USB cable that you first had success with, make sure the USB dip switch is in the ON position. Use Windows Device Manager to look at your USB devices to see if it is even registering that an unknown device is connected. Some users report that using an older version of the drivers from SiLabs is more reliable with Windows 10 and fixes this issue.