Why is the password recovery email not arriving?

I lost the password of my account that I have been using for about 7-8 years. I want a password recovery email but the recovery email is not coming to my email account. I sent an email to all of GrabCAD's emails stating that it has not arrived but I have not received a response. When I re-register with a different email account and request a recovery email, it comes but my old account's email is not coming. Why? How can I fix it?