Many iPhone users initially face no issue with Siri but with time or latest iOS upgrades the Siri does not seems to good job in detecting your voice and voice commands. In case you also want to reset Siri to learn your voice again and update its data base it is simple step to do:
To reset the Siri open o Settings > General > Siri&Serach and toggle the Siri option to Off. After this go to Settings > General > Keyboards and turn off “Enable Dictation.” This will clear all Siri data and command. Now after sometimes you can go back and tunr ON both services.
If this does not seems to work like what you expect you can:
Reset iPhone Settings
It will wipe out all your data at once so please don’t forget to backup your iPhone data on iTunes or iCloud. Settings app on iPhone. Go to General and select Reset. Select Reset All Settings and enter the passcode.