If your peaks are set at 1, or nearly so, what is the value of a pixel representing a lowland area? The relationship between value and elevation in a heightmap is linear, so if you want your highest peaks to be 20,000 feet (255), and the Shire at maybe 3,000 feet, then the value of a pixel in the Shire is only ~38. In floating point space (running from 0 - 1), it's only .066.

What bit depth are you working at? 8-bit gives you only 256 values, which is really not enough to describe the difference between mountain peak and lowlands, at least not if you want a decent amount of detail in the lows. Unfortunately, Photoshop disables a lot of tools in 32-bit mode, so it isn't easy to use.

Something you might try is to run the Lighting Effects on the mountains and lowlands separately. Set up a layer where 0 is around 4,000 feet or so and use your entire range to just put detail in the mountains. Then you can use the full range to get plenty of detail in your lowlands, too, and from there it becomes a matter of finding a way to blend the two layers together.