Are you starting the game via the MODS menu, pressing "NEXT" in the mods menu, then "SINGLE PLAYER", then selecting the map, and then clicking the little round "Load Scenario" button in the modded games game-start menu before clicking (I think it is called) "START SCENARIO" instead of the usual "START NEW GAME" button ?
If you aren't following this procedure, you can select the map in the unmodded game menus but you will get a random start placement with the usual units.
If you do everything in this procedure but forget to click the little "Load Scenario" button on the modded new game regular menu you will also just get a normal and randomly-selected start position on the custom map.
The start positions on a premade map aren't 100% random -- the game will always pick the same X spots but you will not really have any control over which of the spots the game assigns to you.
If you go through the MODS menu but forget the "Load Scenario" step as I recall you will got one of the pre-placed start positions on the map as made in WorldBuilder (if any start positions were assigned) but as best I can recall you will also just be more or less randomly placed for which of the pre-determined start positions you will get.