There's a bug in ElmerGUI: in Heat Equation or Advection Diffusion, if you set Convection = Constant, and set the Convection Velocity, then quit and restart and reload, it will still have Convection = Constant, but the Convection Velocity will not be visible -- and will not be created in the SIF file.
I think this is probably a more general bug involving the "Activate" tag in the equation .xml files, but haven't checked with other Activate variables. Will investigate a patch when I get some time.