loop problem

[fast crystal]

so I wanna find the largest prime factor and the loop kinda stops at some number (pls dont give optimized code rather tell me why the loop doesnt continue)

code:

import java.math.BigInteger;

public class Main {



    public static void main(String[] args) {


        BigInteger num = new BigInteger("600851475143");
        BigInteger erg = BigInteger.ZERO;

        for (BigInteger i = BigInteger.ONE; i.compareTo(num) <= 0; i = i.add(BigInteger.ONE)) {
            if (isPrime(i) && num.mod(i).equals(BigInteger.ZERO)) {
                erg = i;
                System.out.println(erg);
            }
        }

        System.out.println(erg);

    }

    static boolean isPrime(BigInteger num)
    {
        if(num.compareTo(BigInteger.ONE) <= 0)
        {
            return false;
        }
        for(BigInteger i = BigInteger.TWO; i.multiply(i).compareTo(num) <= 0; i = i.add(BigInteger.ONE))
        {
            if(num.mod(i).equals(BigInteger.ZERO)) {
                return  false;
            }

        }
        return true;
    }
}

this is my output and its still in the loop:
https://cdn.discordapp.com/attachments/1116821865772626020/1166473603194310766/image.png?ex=654a9e33&is=65382933&hm=6c4a6f8141d5665421ccc7d113ef26fbb3deaa7e5972cb7efb00c57a906c5ee3&

[lunar zenith]

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[winged cave]

The loop continues, it's kind of the problem. You've found all prime factors so there won't be anymore, but it checks all numbers until your chosen one