DNA: Despite SPG`s letter, how PM`s security get breached?

After the breach in the security of the Prime Minister, many questions are being raised. During this, a letter from SPG also surfaced which was written to the DGP of Punjab. According to the information received, an alert related to the security of PM Modi was given in this letter. In such a situation, it is shocking that despite this alert, how did this security lapse happen.