I have 3 ways of getting audio into my car sterei, headset line out, Bluetooth to line box and Bluetooth to fm.
With all 3 options Alexa hears her name. Only with the Bluetooth to fm module does alexa actually "hear" the rest of the command.
I thought maybe the app is looking for a different mic source but as the initial "alexa" is heard this can't be the case.
The fm module works but the audio is nowhere near as good as the Bluetooth the line in module.
You don't say what is providing Alexa. It is on your phone (the Alexa app), Echo Auto, etc.?