Hmmm, alright...There was a game i played where suddenly i couldn't select the country my army was on, so i couldn't keep attacking...ended up losing cause of it.
It may be bacause you ran out of armies to attack, or there was a barrior (IE: water, mountain, wall,) or "one-way" passthrough preventing you from attacking that other country......Tell me your game number, country, and the country you attempted to attack, and I may be able to explain better.
"You can't rule the world in hiding. You've got to come out on the balcony sometime and wave a tentacle"
It's already cleared out, but, it was the great lakes one, and i was attacking one that was surrounded completely by other countries to attack yet. So, i'm getting that the only reason that i couldn't attack was a bug.