I should have asked earlier - Hub version (C5, C7, C8?) and platform version? Current Hub platform version is 2.3.8.134. If you aren't on that version you should update.
I use the Chromecast integration multiple times every day for door/window open announcements, goodnight tasks, etc., and it always works unless I'm having problems w/my network, GH can't connect for some reason. Not sure why it would not be equally reliable for you.
There is a process to re-initialize your Chromecast via rule - See the example below. You can set it to run as often as it seems to need in your environment - below would run the Initialize daily at 7am.
Start a new rule and set the Trigger to something like running daily in the morning.
For the Action choose the "...Run Custom Action" type:
Set as below:
Rule should look like this:
See if that helps.
Also, when asking for help is helpful if you post your hub version (C5, C7, C8) and current Platform Version you're running on the hub. 
