if ants sit at entrance of nest they will or can get stuck, at that point if your doing gateway mission you will no choice but to restart mission. here is a link to lvl 1 gateway showing this senerio, i had to exit game at that point cuz ants dont know what to do ...
https://www.youtube.com/watch?v=NeZr00xfK48
https://www.youtube.com/watch?v=NeZr00xfK48