This occurs when you did not properly install all the areas for your state. To identify which tiles you missed simply go into your Control Panel and select Add/Remove programs (XP) or Programs and Features (Vista onwards) and look at your list of installed MegaSceneryEarth areas. You will be able to identify the missing missing area by the missing number in the sequence. Then simply install the missing area(s)/tile(s).
The only exception to this is a region in Nevada around Area 51 where no imagery is available most likely due to the classified state of this area.