ELTs are not infallibe. The MH one may not have activated automatically (if the circumstances required its activation) and even if it did, there's no guarantee it worked as advertised. If it is under water it maybe shielded or the antenna / skin where it is mounted may have been torn away in any catastrophic event. The ELT maybe transmitting into a short length of coax that could be physically shorted or now water logged. Some signal maybe transmitted but it would be highly compromised as they are pretty low power anyway at about 5 watts. All speculation of course but a few things that may account for non receipt of a distress beacon signal.