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It has been a year since my mother went for her ceremony, she received her red passport but did not get her nationality certificate delivered. We contacted home office few times and they said the post was lost that's why you did not get it. After that, we contacted and emailed few times, did not hear any respond.

Now I want to know what is the action we should take? Who should we contact for it?

Do you mean HM Passport Office lost the certificate or did you mother not get a certificate? Your post is not very clear.

She should have received the certificate at the ceremony.

So to be clear, your mother attended her citizenship ceremony where she was presented with her certificate. She then applied for her British passport and enclosed the certificate with her application. The passport arrived, without the certificate.
As she now has a passport as proof of her British citizenship, she may no longer need the certificate. However, although the loss may not be her fault, the simplest way may be to pay a fee with a request for a replacement.
https://www.gov.uk/get-replacement-citi ... ertificate

I believe that the HO Passport Office advise the payment of £3 if the applicant wants to have the documents returned by secure post. This means that they are unlikely to take responsibility of any loss if this wasn't paid.

The naturalisation certificate may be more important than a passport.