Why can't I lay an ambush?

[KingDaryoush]

On the campaign map, when I try to ambush an enemy by moving my army into a forest area, the general dude just won't kneel and say "Ready to ambush!", btw im not standing next to any enemy armies.

Also, it seems that if I split my army into smaller ones, I can lay an ambush? so is my army to big or something? or is this a bug?

[Brusilov]

It may be the characteristics of some of the units in the army that prevent the ambush from being set up.

In the file export_descr_unit.txt there needs ot be an entry like this:

Spoiler Alert, click show to read: 

type lucani pedites extraordinarii
dictionary lucani_pedites_extraordinarii
category infantry
class spearmen
voice_type Heavy_1
soldier roman_samnite_warrior, 50, 0, 1.158
officer greek_standard
attributes sea_faring, hide_forest, can_sap, hardy, mercenary_unit
formation 1, 1, 2, 2, 4, square
stat_health 1, 5
stat_pri 11, 12, no, 0, 0, melee, blade, piercing, spear, 25, 0.275
stat_pri_attr light_spear, spear_bonus_6
stat_sec 11, 8, no, 0, 0, melee, blade, slashing, sword, 25, 0.292
stat_sec_attr no
stat_pri_armour 5, 22, 5, metal
stat_sec_armour 0, 1, flesh
stat_heat 2
stat_ground 0, 0, -1, -1
stat_mental 20, normal, trained
stat_charge_dist 32
stat_fire_delay 5000
stat_food 60, 300
stat_cost 0, 1511, 469, 228, 304, 189
ownership barbarian, nomad, carthaginian, eastern, parthia, egyptian, greek, roman

[KingDaryoush]

ye I got hide_forest , I can setup an ambush though, when my general sets up one by himself, and the army joins him, any other suggestions?