Troubleshooting guide
A private message exchange is unavailable
Verify the recipient, your connection, and reception settings, without assuming a private reason for a refusal.
Verify the account and network
Make sure you're using your own account and writing to the correct nickname. A conversation with yourself isn't an exchange between two people. Identical nicknames on two IRC networks don't necessarily refer to the same member.
An exchange between Dialova accounts and a private conversation with someone from an external network doesn't follow the same rules. Durable functions of the site aren't automatically available to people from another network.
Understand reception settings
In Settings → Chat, the option to Block unwanted private messages can limit new contacts. Opening a private message always allows you to receive responses from that person according to the site's access controls.
A block, an inaccessible account, or a restriction can also prevent interaction. The unavailability message doesn't reveal which of these cases applies to another person. Don't repeatedly contact them to determine this.
The person isn't in the list of connected users
A Dialova account can receive offline durable messages when access to that exchange is allowed. Their presence or absence in the list isn't enough to diagnose a problem. A call, however, requires that people can receive it at that time.
If a confirmation or restriction is displayed, read it before proceeding. An absent 'Seen' status doesn't prove the message is lost: sharing of reading may be disabled.
You aren't receiving a new private message
Check your reception setting and the list of blocked members in Settings → Chat. Also, look at conversations and their unread messages: an absent notification and an absent message are two different things.