First, make sure you have reviewed our general speech recognition tips.
In the Speech Recognition (Advanced) section of the settings, you will find some options you can adjust. Here, try enabling or disabling Google speech recognition or the use of Bluetooth. If the app is telling you that it isn't hearing anything, try lowering the Voice Activity Detection Threshold.
Unfortunately, for some users (especially those with non-US accents), it may not be possible to get high accuracy with speech recognition. In this case, we recommend using the option to include an audio recording of your voice with outgoing messages. By playing the recording, the receiver of the message will be able to understand what was said even if the text is not clear. You will find the option to turn this on in the Outgoing Message Options section of the settings.