There are several possible reasons why this might be. First of all, try the following:
- Check you have an active Xbox Live Gold Membership. You cannot use the Matching feature or play online without one.
- You cannot use the Matching feature if your Lobby is set to "Private." Please check your Lobby settings are set to "Open." If it reads "Private", push the button to switch to "Open."
- The Matching feature will be unavailable if you have been given a disconnection penalty. You can check whether you are still in the penalty period by looking at the dialogue box that appears when you enter the Lobby. You must wait until the penalty period ends before you can use Matching again.
- Your NAT settings can also cause connection problems. In your Xbox One network settings, check your NAT settings are set to "Open."
- There may not be any other players in the dungeon you have chosen. Try selecting a different dungeon and attempt Matching again.
- Network problems can cause the Matching feature to become temporarily unavailable. Please check your network connection, then wait a while before trying again.
If this does not solve the problem, please fill out a support form, clearly describing the issue.