That used to happen to me when Do Not Disturb was "ON" on my iPhone. When this feature is enabled, calls and alerts that arrive while the screen is locked will be silenced and a moon icon will appear on the status bar.
Can you go to Settings - Do Not Disturb - Manual and check if the feature is on? You'd also have to check the Allow Calls From setting as well as the Repeated Calls.
If your Do Not Disturb is set to not accept calls from anyone or only accept calls from Favorites and the Repeated Calls feature is on, then only calls received from the same person the second time would ring and not be sent to voicemail (provided that the caller is not part of your Favorite list).
Let me know if this helped