My understanding is still imperfect, but I'll try to provide some info:
Not all messages are encrypted with the same key, so if all of your clients are not connected at the same time, and the same is true for the sender, they can't exchange their keys. When that happens, each client can only decrypt the subset of the messages for which it has the keys. Also note that clients only exchange their keys with other verified clients.
If you look at the “session_id” attribute of the JSON source of the messages, you'll see that for a given session (ie. when the sender is logged in a client), all the messages are decrypted (which means you have the key for that session) or none of them are (which means you haven't received the key for that session yet).
Not all messages are encrypted with the same key, so if all of your clients are not connected at the same time, and the same is true for the sender, they can't exchange their keys. When that happens, each client can only decrypt the subset of the messages for which it has the keys. Also note that clients only exchange their keys with other verified clients.
If you look at the “session_id” attribute of the JSON source of the messages, you'll see that for a given session (ie. when the sender is logged in a client), all the messages are decrypted (which means you have the key for that session) or none of them are (which means you haven't received the key for that session yet).