Can someone explain why my luhn's algo fails?

#include <math.h>
#include <stdio.h>
#include <stdbool.h>
#include <cs50.h>

short unsigned int numlen(unsigned long long n);
bool isValidCC(unsigned long long int n);
unsigned short int first_n_nums(unsigned short int n, unsigned short int x);


int main(void) 
{
    unsigned long long num = 0;
    num = get_long("Enter your card number ");
    
    if(numlen(num) == 15 && (first_n_nums(num, 2) == 34 || first_n_nums(num, 2) == 37) && isValidCC(num) == true)
    {
        printf("AMEX\n");
    }
    else if((numlen(num) == 13 || numlen(num) == 16)  && first_n_nums(num, 1) == 4 && isValidCC(num) == true)
    {
        printf("VISA\n");
    }
    else if(numlen(num) == 16 && (first_n_nums(num, 2) == 51 || first_n_nums(num, 2) == 52 || first_n_nums(num, 2) == 53 || first_n_nums(num, 2) == 54 || first_n_nums(num, 2) == 55)  && isValidCC(num) == true)
    {
        printf("MASTERCARD\n");
    }
    else
    {
        printf("INVALID\n");
    }

    
}

unsigned short int first_n_nums(unsigned short int n, unsigned short int x)
{
    while(n > pow(10,x))
    {
        n /= 10;
    }
    return n;
}    
    
   

short unsigned int numlen(unsigned long long n)
{
    short unsigned int count = 0;
    while (n !=0)
    {
        n /= 10;  //removes last digit
        count++; //counts the num of digits removed
    }
    return count;
}

bool isValidCC(unsigned long long int n)
{

    // take user input and add elements to array in reverse
     int arr1[(numlen(n))]; // declares but can't Initialize an array of n ;
     
     for (int i = 0; i < (int)sizeof(arr1)/sizeof(int) ; i++)
    {
        arr1[i] = 0;
    }
    
    for (int i = 0 ; i <(int) (sizeof(arr1)/sizeof(int)) ; i++)
    {
        arr1[i] = n % 10; //Appends last digit to an array
        n /= 10; // removes that last digit
    }
    
    
    // since arr1 = x digits long, arr2 is every second digit so its x/2 long 
     int arr2[((sizeof(arr1)/sizeof(int))*2)];
    
    //initializing garbage data to 0
    for (int i = 0; i < sizeof(arr2)/sizeof(int) ; i++)
    {
        arr2[i] = 0;
    }
    
    // multiplies, seperates, and sums arr2 elements
    int sum_of_arr2 = 0;
    for (int i = 1,  a = 0,  b = 0 ; i < (sizeof(arr1)/sizeof(int)) ; i += 2) // sizeof(array) = n of elements * sizeof(type of n)
    {
        a = arr1[i] * 2;
        if( a > 9)
        {
            b = a % 10;
            a /= 10;
            arr2[i] = b;
            sum_of_arr2 += b;
        }
        arr2[i-1] = a; // cz i currenrly is one idex ahead
        sum_of_arr2 += a;
    }
    
    // adds other elements of arr1 to sum_of_arr2
    for (int i = 0 ; i < (sizeof(arr1)/4) ; i += 2) // sizeof(array) = n of elements * sizeof(type of n)
    {
        sum_of_arr2 += arr1[i];
    }
    
    // returns true or false
    if (sum_of_arr2 % 10 != 0)
    {
        return false;
    }
    else
    {
        return true;
    }
}

Can someone pls explain why this fails for values like:

371449635398431 - expected "AMEX\n", not "INVALID\n"

5555555555554444 - expected "MASTERCARD\n", not "INVALID\n"

5105105105105100- expected "MASTERCARD\n", not "VISA\n"

4111111111111111 - expected "VISA\n", not "INVALID\n"

4222222222222 - expected "VISA\n", not "INVALID\n"