Speech to Text Returning Empty string for displayText

Question

audio_file=@'9a043053-a1ad-4ebd-861a-c85426697147.wav'

curl --location --request POST \
"https://eastus.stt.speech.microsoft.com/speech/recognition/conversation/cognitiveservices/v1?language=en-US&format=detailed" \
--header "Ocp-Apim-Subscription-Key: " \
--header "Content-Type: audio/wav" \
--data-binary $audio_file


-- getting DisplayText empty for most of the audio files 
{
    "RecognitionStatus": "Success",
    "Offset": 300000,
    "Duration": 301200000,
    "DisplayText": ""
}