In theory, your counsel should be handling this stuff, in reality, you have to make sure they are. If they're not doing an adequate job, complain to the insurance company and have them replaced.

Sending notifications to the doc offices out via certified mail return receipt requested might not be a bad idea even.

Did the ALJ make an error in the denial?

You can submit a new application, btw. It might cost your insurance company the lost backpay, but that doesn't seem to be your fault...