Make sure the troops can reach the signpost from the very edge of the map. If you put your signpost on an island, or block the edge of the map with rocks or something, your invasions won't appear. I believe that the invasions will try to show up at a certain point on the edge of the map based on which signpost the invasion uses, and even if there is a clear path from other points on the edge of the map to the sign post, the particular point that the invasion tries to come from might be blocked. I haven't tested that particular theory, but I can definitely say that if your map is an island that will be why your invasions aren't coming.
Additionally, I'm assuming this is what you mean by "setting up the waves," but in case I'm wrong, make sure that you've adjusted the number of troops in the invasions to be greater than zero. Also, if the forces that show up are small enough, the computer will retreat immediately, before the troops even show up, so you might try increasing the number of attackers.
If none of that works, you might try sending it over to me so I can take a look at it.
-DOOMBRINGER-
"DOOMBRINGER, make a parking space IMMEDIATELY!"
"DOOMBRINGER declares it to be SO!"
"DOOMBRINGER, I command you to lay siege to THE ISS!"