Did you check both cards slots to see if the issue is maybe a faulty card reader?
Is your Navionics card older than 2005?
Did you corrupt the card by using it for other than the maps that are on it?
My ONIX 10 SI reads every card I have, Lakemaster, 7-Navionics, and other cards for software updates, recordings, snapshots, and waypoint management.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1