Holding a key, then pressing another, stops original key being pressed

Question

(No, not a hardware issue)

I have coded kbhit() in standard C as follows (obtained from here; another function "setTerm()" sets (non)canonical mode):

#include <sys/ioctl.h>

int kbhit()
{
    int bytesWaiting;
    ioctl(STDIN_FILENO,FIONREAD,&bytesWaiting);
    return bytesWaiting;
}

And I have subsequently implemented kbhit() in a program that reads and prints the keyboard input to the screen. When a key (how about 'A'?) is held, it is repeatedly printed. However, when a second key (say, 'B') is pressed while 'A' is held, 'B' is printed and no more 'A's appear. My intentions are that 'A' continues to be printed, but this is not the case. Here is some code showing the issue (meant for use with arrow keys or A-D; press Enter or '\n' to end):

#include <stdio.h>
#include "kbhit.h" //just replace this line with previous code example

int main()
{
    setTerm(0);//turns terminal to noncanonical mode; if necessary I can include the function

    int test=1;
    int c=0;
    while(test)
    {
        if(kbhit())
        {
            c = getchar();
            switch(c)
            {
                case 'A':
                case 'B':
                case 'C':
                case 'D':
                    printf("%c\n",c);
                break;
                
                case '[':
                    printf("%c",c);
                break;

                case 27:
                    printf("ESC");
                break;

                case '\n':
                    test=0;
                break;
            }
        }
    }

    setTerm(1);//restores terminal to canonical mode
}

Again, I can add my setTerm() function if necessary. But the question remains, how can I keep taking the input of the held key?