If the stand mixer only runs on the highest speed but not on medium

The speed issue could be related to the spark suppression capacitor for the governor. If it shortens out the motor will run at full speed only. The capacitor is located under the Bakelite cover that covers the motor bottom. Remove the cover and you should see the capacitor. You can always post images of what you find with your question by using this guide Aggiungere immagini ad una domanda That way we can see what you see.

If that capacitor is good you can try to adjust the speed by using this guide which should be similar to all Kenwood's. Like @blakeklein already mentioned, let us know what exact model this is so we can help you further if needed.