I think you haven't understood how ambush works.

That is not an abush i fear
That force can't ambush as a whole. Only the light horse element there (partisans i guess) can set ambushes. Its limited to irregulars type of troops.
The rest of the guys are doing a normal battle, in offensive for what i see again trenches...
To ambush, a 100% irregular force should start the turn already in the region, be ordered to set an ambush and let the enemy come to them (if they fancy to pass through that region

), not the other way aroudn. And it's not even guaranteed the trap will work
My advice would be: forget about ambushes in AACW. They rarely work and are not that effective.
In WIA, with all that wilderness and Indians units around and lots of smaller scale battles they are more useful, but you can't expect miracles either.
Cheers!