If you would do a little search before posting, you'd know it's known bug and is being worked on...
Wow what a helpful response. If I ran a company I'd hire this guy to run my Customer Service in a heartbeat.
Seriously though, it is an annoying bug but here is what I do when I get it:
I place guys around the central stairs all on RF.
Have one guy shoot up the stairs (doesnt need to be at an alien, just a wall).
Move the shooter behind a wall.
Usually some sort of noise, like a shot, will get them to come down the stairs. Dont use a grenade
because if you miss you might hurt someone. I ended up blowing 3 of my own guys up doing that.
If that doesnt work, sometimes having your entire crew exit the UFO can get them to come down.