Army can't enter caves? Only one company per turn?
When I try to enter a cave with dwarves blocking the other end only 1 company goes in every turn and basically gets killed. .the rest of the army doesn't go in with him even though they are all in the same army.
Same thing with the AI dwarves. They only attack with 1 company every turn even though they had an army bigger than mine waiting on the other end.
Is this a bug?
No, this is not a bug. Due to the limited space in the tunnels connecting the Surface and the Netherworld only one company at a time can attack through cave openings. The company with the best attack value (strength*attack value) will be selected for the attack. Likewise only one company will defend from the attack, selected from the defending armies in a similar way using the defensive value instead of attacking value.
If you want to wear down your enemy on the other side of the cave opening I suggest attacking every single turn using single company armies to make sure that you can keep up the pressure.