Things that impact troop production:
Barracks - faster production times
Armouries - more troops per training slot in barracks
Wonders - certain wonders give free troops or increase training speed or increase training size
For stronger armies:
Squad Size technology
Wonders - Those that increase damage or HP
For weaker enemies:
Advanced scout tech - Decreases the enemies strength for unscouted provinces
Scouting approach - Again not directly affecting your troops but the enemy strength.
Scouting in a "circular" fashion, difficulty jumps a lot when you go an extra ring distance from your town
Timing of scouting to your advanced scout tech. Minimal or moderate scouting will have enemies you can beat. Scouting as much as possible will mean provincial combat becomes impossible
Chapter - If you are in chapter 1-3 you may not have access to all unit types placing you at a disadvantage. Provincial combat is also unbalanced in this area so just get to the end of chapter 3 through negotiating.
Number of armories depends on your play style. Anywhere from 2-4 is fairly normal for a combat person in most chapters. Some people do have up to 6 armories, later in the game wonders can influence this as well.