I’m unable to fast travel to safe houses we’ve unlocked and field radios I’ve placed down. Not sure why. It worked the first few hours playing but now it hasn’t worked in a day.
And my friend has noticed recently any robots, mainly hunters, we kill will float into the sky.
If the stuff you carry is over the max weight limit, you can’t fast travel at all. Though, you still can move, to some extent. Unless you carry so much where movement is disabled.
Drop enough stuff to get your weight below max, so you can fast travel again. Also, Carry Capacity skill helps since it increases max carry weight, if you don’t have it already.
Well, they are going to the machine heaven. 
Though, this is reported before: Disappearing Dead Robots-Flying Away?