While loop not as expected

This is my Code:

class Warrior:
    def __init__(self):
        self.name="Warrior"
        self.health=50
        self.attack=5
        self.is_alive=True

class Knight:
    def __init__(self):
        self.name="Knight"
        self.health=50
        self.attack=7
        self.is_alive=True

class Army:
    def __init__(self):
        self.units=[]
    def add_units(self, Unit_type, amount):
        for i in range(amount):
            self.units.append(Unit_type)

class Battle:
    def fight(self, army1, army2):
        x=0
        y=0
        while x<len(army1.units) and y<len(army2.units):
          if singlefight(army1.units[x], army2.units[y]):
            y+=1
            print("y:"+str(y))
            print(str(army1.units[x].name)+" wins")
          else:
            x+=1
            print("x:"+str(x))
            print(str(army2.units[y].name)+" wins")

def singlefight(unit_1, unit_2):
    round=0
    while unit_1.is_alive and unit_2.is_alive:
        round+=1
        print ("Round "+str(round)+"\n"+str(unit_1.name)+":"+str(unit_1.health)+" "+str(unit_2.name)+":"+str(unit_2.health))
        attack(unit_1, unit_2)
        print(str(unit_1.name)+" attacks "+str(unit_2.name)+" and deals " +str(unit_1.attack)+" damage")
        print(str(unit_2.name)+" now has " +str(unit_2.health) + " health")
        if unit_1.is_alive and unit_2.is_alive:
            attack(unit_2, unit_1)
            print(str(unit_2.name)+" attacks "+str(unit_1.name)+" and deals " +str(unit_2.attack)+" damage")
            print(str(unit_1.name)+" now has " +str(unit_1.health) + " health")
    return unit_1.is_alive

def attack(attacker, defender):
    defender.health-=attacker.attack
    if defender.health<=0:
        defender.is_alive=False


if __name__ == '__main__':
    # chuck=Knight()
    # chip=Warrior()
    # fight(chuck, chip)
    army1=Army()
    army1.add_units(Knight(), 5)
    army2=Army()
    army2.add_units(Warrior(), 6)
    battle1=Battle()
    battle1.fight(army1, army2)

A lot of print commands for debugging. I use a while loop with iterations for the two army units. During the first loop the battle class function goes into my singlefight function. But any consecutive loops seem to not go there... Why?!? Output:

Round 1
Knight:50 Warrior:50
Knight attacks Warrior and deals 7 damage
Warrior now has 43 health
Warrior attacks Knight and deals 5 damage
Knight now has 45 health
Round 2
Knight:45 Warrior:43
Knight attacks Warrior and deals 7 damage
Warrior now has 36 health
Warrior attacks Knight and deals 5 damage
Knight now has 40 health
Round 3
Knight:40 Warrior:36
Knight attacks Warrior and deals 7 damage
Warrior now has 29 health
Warrior attacks Knight and deals 5 damage
Knight now has 35 health
Round 4
Knight:35 Warrior:29
Knight attacks Warrior and deals 7 damage
Warrior now has 22 health
Warrior attacks Knight and deals 5 damage
Knight now has 30 health
Round 5
Knight:30 Warrior:22
Knight attacks Warrior and deals 7 damage
Warrior now has 15 health
Warrior attacks Knight and deals 5 damage
Knight now has 25 health
Round 6
Knight:25 Warrior:15
Knight attacks Warrior and deals 7 damage
Warrior now has 8 health
Warrior attacks Knight and deals 5 damage
Knight now has 20 health
Round 7
Knight:20 Warrior:8
Knight attacks Warrior and deals 7 damage
Warrior now has 1 health
Warrior attacks Knight and deals 5 damage
Knight now has 15 health
Round 8
Knight:15 Warrior:1
Knight attacks Warrior and deals 7 damage
Warrior now has -6 health
y:1
Knight wins
y:2
Knight wins
y:3
Knight wins
y:4
Knight wins
y:5
Knight wins
y:6
Knight wins