Paul,
If you have Version 7, you might take a look at the Alerts and Warnings in the Setup menu. Set the Collision Alerts to Moderate or Strict, this will pop more alerts. You should see a Red "Halo" around the towers that are close to your altitude. I've attached a picture to give you an example. You'll notice the majority of the screen it yellow, then the some red, the towers are highlighted with a red circle around them.