There is a device reset procedure, but I would suggest calling customer support and they can walk you through it. I had a few sensors that did not want to cooperate when I did my installation, but support got them reset and running. IMHO, one of the best things about Ring, the real person customer support!
If you've added 5 sensors successfully, it sounds like you are following all the proper steps @Radman2050. There are times where the Base Station is in the process of adding or updating a device while another is attempting to add, resulting in a failed setup. The first step is to power cycle the Contact Sensor by removing and reinserting the battery, and try adding it again.
The next best step would be to perform a reset as @Brad stated. Take a look at our help center article for information on resetting your Alarm devices. If this does not resolve the concern, our support team will be more than happy to assist further at 800-656-1918.