This is a strange solution, but I first ran into it using a Roku, and when I had this same issue with my Chromecast, I tried it and it worked - you need to make sure the IP address assigned to the Chromecast device has a full 12 digits. For example, in my case when the address was 192.168.125.6 it would not work, but when I got it an address of 192.168.125.126 it immediately showed up in the cast drop down as an available device. The only way I was able to do this was to change the DHCP scope in my router as I could not find a way to manually assign an address. Hope this helps! It's very weird, but definitely worked for me.