For whatever reason, Moghedien never commanded Myrdraal in any of her schemes, so it is possible that she didn't have any under her direct control due to her tactics.

Also, the Myrdraal still would not know what to do to release the a'dam. It is a rather precise technique, remember? Moghedien didn't know how to do it, and she would need to tell a Myrdraal precisely how to manipulate the collar.

Finally, how would she get the message to them? It seemed to me that the Forsaken don't have a mystical connection to the Shadowspawn they commanded - they literally had to give a command and/or order the Shadowspawn to do certain things, which may have meant Travelling to the Blight where they are bred. I doubt Moghedien would want to appear where any one of the Forsaken could be hanging around in her condition. She most likely didn't even want the Dark One to know. Semirhage was in a less precarious position and even then, an agent of the Dark had to be sent to her, possibly because her mission had failed and her situation had been revealed to Shaiadar Haran by Elza.