Sometimes the bug is right in front of your nose, but maybe you are busy overthinking, being tired, or simply require more time to learn... So take breaks if you want but NEVER GIVE UP!
Not all machines in the world are vulnerable to any specific kind of vulnerability, it all depends honestly. So it will require patience to figure things out
If you are unable to spot a vulnerability in a day, that's completely normal... These things can easily lead to burnout so remember to stay calm and take breaks.