Hi Ann
I experienced the same problem last week. Initially I was passed back and forth between Abbott and Apple. I was told there was recognised problem with this which they were working to resolve. I was guided to use the reader instead. I then went into the local store Apple store locally. After doing some checks they replaced my phone as it was still under warranty.
Abbott were not quick to share there was a known issue, they seemed happy keep replacing sensors.
Hope this of some help to you