To summarize:
(1) If this ship with troops is in a fleet, make sure the fleet's strategy is set to "Capture Planet". If it isn't, then...
(2) Make sure the fleet's strategy is one that allows ships to break formation. (when a ship breaks formation, it reverts to its individual strategy. In this case, the troop ship's individual strategy must be "Capture Planet" (please verify).
(3) And this is the part I'm not sure of: Seems like I've heard somewhere that troops won't be dropped if there are active platforms there. Someone else will need to confirm this.
(My problem is I usually glass the planet before the troops can be dropped
)