Ground combat (tactical) 1.33

What's up with this: Invaded an undefended planet with 50 troops. Defending militia is 4. My troops are stuck except two who get pounded by the militia. After experimentation, I find I can control my troops if I click in certain places (anywhere outside the city and one or two places in the city) and they start moving, but shortly stop again as if they can't find a path to the destination. Firing is unaffected, but I've gone two turns and lost three troop to the defenders. I defeat the militia, but there's always one guy who's damaged and runs back into the city where my troop apparently can't find him. Battle ends in a stalemate and I go to the next turn, except I've lost three troops and he'll get more militia next turn.