Strange issue with input on game engine.

Hello,

I am trying to make a FPS game with a second game engine project that I have been working on. However, I noticed that when I play the game, I can only press down one key at a time, whereas a normal game allows you to hold down multiple keys at the same time (like “Up" and “Left”). This one, on the other hand, only lets me press “Up”, and when I press “Left”, the character would stop moving forward, and only move to the side.

Here is my walking forward code:

…
#define UP 0x4800
…
 int main(void …