Timed images

[menator01]

I've been following a tutorial on youtube "buildwithpython" on a spaced invaders game. tutorial went great leaning alot. I've been tweaking and experimenting on adding different thind. I'm having problems getting my explosion image to appear on every monster hit. It will appear from 1 to 3 times and then nothing for two or three more then repeats.

Relevant code below. if you need more let me know.
Thanks for any help

# Explosion

explosionImg = pygame.image.load('explosion2.png')
explosionX = 0
explosionY = 0
explosionX_change = 0
explosion_change = 0
explosion_state = "ready"


def isCollision(monsterX, monsterY, bulletX, bulletY, explosionX, explosionY):
    distance = math.sqrt((math.pow(monsterX - bulletX, 2)) + (math.pow(monsterY - bulletY, 2)))
    if distance < 27:
        return True
    else:
        return False


#### My addition #####

def explosion(x, y):
    global explosion_state
    explosion_state = "explode"
    screen.blit(explosionImg, (x, y))


 # Monster Movement
    for i in range(num_of_monsters):
        monsterX[i] += monsterX_change[i]
        if monsterX[i] <= 0:
            monsterX_change[i] = 2
            monsterY[i] += monsterY_change[i]
        elif monsterX[i] >= 736:
            monsterX_change[i] = -2
            monsterY[i] += monsterY_change[i]

        # Collision
        collision = isCollision(monsterX[i], monsterY[i], bulletX, bulletY,explosionX, explosionY)
        if collision:
            bulletY = 480
            bullet_state = "ready"
            score_value += 1

            #### My addition ####
            explosion_state = "ready"
            explosion(monsterX[i], monsterY[i])
            #####################################

            monsterX[i] = random.randint(0, 735)
            monsterY[i] = random.randint(50, 150)

            sounds('explosion.wav')


        monster(monsterX[i], monsterY[i], i)