Who cannot acquire the right of subrogation ?

Transfer of Property Act: Section 92.  Subrogation: Any of the persons referred to in section 91 (other than the mortgagor) and any co-mortgagor shall, on redeeming property subject to the mortgage, have, so far as regards redemption, foreclosure or sale of such property, the same rights as the mortgagee whose mortgage he redeems may have against the mortgagor or any other mortgagee. The right conferred by this section is called the right of subrogation, and a person acquiring the same is said to be subrogated to the rights of the mortgagee whose mortgage he redeems. A person who has advanced to a mortgagor money with which the mortgage has been redeemed shall be subrogated to the rights of the mortgagee whose mortgage has been redeemed, if the mortgagor has by a registered instrument agreed that such persons shall be so subrogated. Nothing in this section shall be deemed to confer a right of subrogation on any person unless the mortgage in respect of which the right is claimed has been redeemed in full.