Check that in your emonhub.conf file, there is a section for the Node ID that the emonTH is set to on its DIP switches. You can find out the meaning of the switch positions in the “Resources” section.
Also, depending on the version, there are two different message payloads, one for the V1.5 (with the DHT22) and one for the V2 (with the SI 7201), and the data structure defined in emonHub.conf must match the payload for it not to be rejected.
Nodes 19 - 26 should be present in your emonHub.conf, and those cover the standard variations.
One point to mention: your two emonTHs must have different NodeIDs.
Can you be a bit clearer with what you mean by
Have you looked at the received signal strength (rssi) on the Inputs page?