Speech-to-text has detiorated

I just one week ago purchased the iPhone 8 Plus. For the first couple of days, the speech recognition was flawless. I was so very impressed and pleased! But then, the phone began making rather serious misinterpretations of my speech. My articulation and enunciation are very good. I didn't have problems with my old phone, the Samsung GS5. Anyone have an idea why the sudden change after such a wonderful beginning with this phone? Thanks.