I do not recall seeing this before and cannot reproduce the problem in the siege window. Are you sure that you have selected the Assault option before attempting to move your infantry? You do not need to move artillery into the front line box to bombard, just press the Bombardment button and then E...

On the siege screen, as the attacker, I am not able to deploy my troops. Every time I try drag and drop an infantery corps in to the frontline, ww1 tells me that only infantry corps are allowed to be deployed there. Same for artillery, so I cannoct conduct any assaults or bombardments of forts. This...