CarPlay sometimes just doesn't connect or start? ...

2018 Subaru Crosstrek with CarPlay. It requires that the phone be connected via USB/Lightning cable. The overwhelming majority of the time simply connecting the phone will make CarPlay available. But once in a while it doesn't. In that case I disconnect and reconnect the iPhone's (iPhone X, previously 7+) Lightning cable and it works.

But once in a while it doesn't. Today I restarted the car, and unplugged/reconnected the phone, and CarPlay never became available.

Using iOS 11.1.1. Any ideas as to what's happening here?

Thanks in advance!