Need more information on how you're actually doing it to be able to offer any useful advice.

All I can suggest is that you ensure you follow the instructions in the accompanying readme files.

Not all scenery can be installed the same way. Some can go in the addon scenery folder, some needs to be placed in the scenery folder. It all depends on how the bgl files have been written and where they're drawing the textures from.

Perhaps you could give a file name of something that isn't working and a description of how you installed it?

Try not to send it to the bottom of the list.  Something above it maybe overwriting it....  Leave it at the top when you add the area.