My Iridium 9555 or 9575 handset can get a good signal off the cradle but when docked, I get intermittent signal and sometimes no signal at all.

The Iridium 9555 and 9575 handset have an external bypass antenna connector. As these connectors are exposed to the environment, they can get dirty and may prevent a good connection between the handset and the dock. Sometimes, the more use the handset gets with that connector, it cannot be avoided that the connector will wear off and may not offer good connectivity to the dock.

The antenna connector of the 9555 handset is at the back just above the battery while the 9575 handset is at the bottom.

Make sure the connector is clean and free from any foreign matter that may block the connection. Connectivity of the external connectors can also be verified with an accessory that came with the handset.

The 9575 comes with a bottom connector that will allow you to use an external antenna while the 9555 comes with a clip adapter for the same purpose. If using the adapter still gives you no signal, then have your handset checked by your supplier or by an Iridium authorized service centre.