Agree with remove and reinsert. If that doesn't work, reboot the iPhone by holding in the Home and Sleep/Wake button together for 10 seconds and then letting go.
I Have the same problem with my iPhone 6. I found if I turn it off and back on again, it works fine for a while, obviously not a good situation. I am going to try and reinsert it to see my f that permanently solves the proble,else off to the store I go.