Are you using a wired headset or third party Bluetooth device? Sometimes Cortana can't hear me when I say read a message when using the hands free device on my Jeep. The fix around that is to open the Bluetooth settings and check the box to "use alternate Bluetooth audio connection for speech."
Second thing, is Cortana available in your region? If so is it able to configured to understand different accents and dialects?
Third, if not using anything external, is your microphone clear of obstructions or anything that would impede or interfere with your voice not being heard? I hope I was able to be of some assistance, wish you the best.