system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter]How come the Ministry of Magic can get a letter in seconds of underage magic being performed , but have no idea that a fight is going on in their own building for like one hour?

I'm referring to the letter after dobby dropped the cake , and of course the battle where dumbeldore and Voldie fought , and the reinforcements arrived eventually.
gpt
I can't imagine He Who Must Not Be Named could've just waltzed into the Ministry of Magic at any given time. The fact that he was there leads me to believe he subdued or killed those who shouldve been the first responders, which is why they took so long to get reinforcements. 

On a separate note, wizards no longer have their magic usage tracked once they come of age. The Ministry doesnt get notified of a fight if both participants are of age, let alone if the participants are He Who Must Not Be Named and Professor Dumbledore.