An F.I.R can only be filed for a cognizable crime. In the event someone is trying to file an F.I.R for a crime that falls in the non-cognisable category it is the duty of the police to listen to them, enter the matter in their daily register or dairy, give the person a signed copy of the entry made (as proof of the matter being recorded) and direct them to the closest or appropriate magistrate. The signed copy of the entry made by the police is free of cost and is a right to receive.
The police may not investigate a complaint even if you file a FIR, when: (i) The case is not serious in nature; (ii) The police feel that there is not enough ground to investigate. However, the police must record the reasons for not conducting an investigation and in the latter case must also inform you.