This sounds very odd, Dubrok.
What should happen, and happens for me without fail, is the troop transport will head for a corner as long as there are weapon platforms on the planet. Meanwhile your attack ships will attack the planet and eliminate the platforms with hardly any damage to facilities or pop. or other items in cargo.
When the platforms are gone, the transport will close on the planet and drop its troops. With the platforms gone it is in no danger from the planet but is still vulnerable to fire from sats, fighters and ships, though your attack ships should still be attacking those. If no enemies are left your ships will either head to the corners or park in place which usually gives your transport room to get to the planet.
I have no explanation for your ships retreating with active enemy defenses in place. ... Ah! An idea! Was it possisble all the remaining ships had all their weapons destroyed?
And yes, 400 troops is plenty for 1.6 million pop, unless he too has 400 troops!
