UAVs can only be used if you have the right terminal -
Eg, NATO UAV needs a NATO terminal.
However, in the virtual arsenal, all the terminals are called 'UAV Terminal' (I can't remember if the UAV backpacks have a similar naming convention).
This means if you have a NATO UAV, you have to equip the first backpack, try it out, if it doesn't work try the second, if that doesn't work try the third. Then repeat the process for other faction UAV operators.
It gets frustrating - can't they just be named:
UAV Backpack (NATO)
UAV Terminal (NATO)
UAV Backpack (CSAT)
UAV Terminal (CSAT)