Welcome to Apple Support Communities. I understand that you're having trouble using Dictation on your iPhone 6 Plus. I know how convenient it is to use this feature; I'd like to help. First I'd suggest checking that dictation is enabled in Settings > General > Keyboards > Enable Dictation. If it is currently enabled, disable it momentarily and enable it again; test to see if Dictation is working as expected.
If you continue to have trouble could you let me know what happens when you try to use the microphone to dictate? In the Messages app do you see a voice recording instead?