Having a character shooting bullets in Pygame

Question

I am trying to create a game in pygame that involves one character shooting bullets at enemies on the screen (a bullethell type). I have created classes for the main protagonist and for the main enemy and have them all on one main script.

import pygame
import numpy as np
from apple_class import *
from snake_class import *
from explosion_class import *
from bullets_class import *

score = 0
pygame.init()
screen_width = 640
screen_height = 400
FPS = 40
screen = pygame.display.set_mode((screen_width, screen_height))
clock = pygame.time.Clock()

snake = Snake()
apple = Apple()
explosion = Explosion()

apple_size = 12
snake_size = 8
bulletlist = []

def CollisionDet(x1, y1, x2, y2, size1, size2):
    if x1 >= x2 and x1 <= x2 + size2 or x1 + size1 >= x2 and x1 + size1 <= x2 + size2:
        if y1 >= y2 and y1 <= y2 + size2:        
            return True
        elif y1 + size1 >= y2 and y1 + size1 <= y2 + size2 or y2 >= y1 and y2 + size2 <= y1 + size1:
            return True


running = 1
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            running = False

    score += 0.1
    key = pygame.key.get_pressed()

    if snake.hit_points > 0:
        snake.handle_keys()

    screen.fill((255,255,255))
    snake.draw(screen)

    ##VARIOUS APPLE MOVEMENTS##
    if apple.x <= 0 or apple.x == 0 and apple.y <= 0:
        side = 1

    if apple.y <= 0:
        apple.y = 0

    if apple.x < 620 and side == 1:
        apple.move_par()
    elif apple.x >= 620:
        side = 2
    elif apple.y >= 380:
        side = 3

    if side == 2:
        apple.move_par_right()

    if side == 3:
        apple.move_par_bot()

    apple.draw(screen)

    if CollisionDet(snake.x, snake.y, apple.x, apple.y, snake_size, apple_size):
        snake.hit_points -= 1
        print(snake.hit_points)


    ##FIRING BULLETS##
    if key[pygame.K_SPACE]:
        bullet = Bullets(snake.x + 5, snake.y)
        bulletlist.append(bullet)
    for b in bulletlist:
        b.move()
        b.draw(screen)

        if CollisionDet(b.x, b.y, apple.x, apple.y, 5, apple_size):
            apple.x = 0
            apple.y = 0


    ##GAMEOVER, BLIT EXPLOSION IMAGE##
    if snake.hit_points <= 0:
        print('you lose, dingo')
        explosion.x = snake.x
        explosion.y = snake.y

        explosion.draw(screen)


    pygame.display.update()

    clock.tick(FPS)

I am able to get the main character (snake) to fire bullets at the constantly moving target (apple) when I press space. The problem is a very strange one however, whenever I move diagonally upward as the snake, I cannot fire bullets at the same time. Here is the snake class:

import pygame

pygame.init()

class Snake():
    def __init__(self):
        self.image = pygame.image.load('snakehead2.png')
        self.x = 320 #width/2
        self.y = 200 #height/2
        self.hit_points = 5

    def handle_keys(self):
        #GET_PRESSED RETURNS TRUE OR FALSE VALUES
        key = pygame.key.get_pressed()
        dist = 5
        if key[pygame.K_DOWN]:
            self.y += dist
        elif key[pygame.K_UP]:
            self.y -= dist
        if key[pygame.K_RIGHT]:
            self.x += dist
        elif key[pygame.K_LEFT]:
            self.x -= dist        

    def draw(self, surface):
        surface.blit(self.image, (self.x, self.y))

Essentially, whenever I press K_UP and K_LEFT and K_SPACE all at the same time, the space button does not seem to render. Is there something wrong with the snake class or the main code? Thanks for the help.