Hi,
Did you say all enemy units are visible?
When? While your air assets pass over them?
Did this happen all the times, or did this happen only once?
Can you reproduce it (if yes please attach a save of the game)?
Also which is the scenario you are playing, maybe I will be able to give you a hint on how to advance.
If you wish to advance through an open field then you should first advance your light infantry units and/or scouts. It would also be best if you create with your artillery a smoke screen just to prevent the enemy of doing shooting practice against those recon units. Also move them with 1/3 to 1/2 of their speed in order to make a cautious advance. Finally try to move them through forest or rough hexes to gain cover and defensive bonus.
cheers,
Pyros