There are two reasons why your Raspbian might not recognise the SDRAM on top of the SoC for older PI's, or on the back of the board for PI3's.
- The SDRAM is actually defective, which is very rare, as all PI's leave the factory working, but can still not be simply ruled out, as the defect can develop later, for example by dropping the PI, or damaging traces on the board.
- The type of RAM used is newer than the "BIOS -like" part of the GPU boot software that is part of the boot code in the FAT32 partition on the SD-card supports, so the GPU code doesn't know how to initialise the SDRAM for use.The latest software is guaranteed to know all previously used types of SDRAM, and the software will be available at least weeks before selling PI's with a new type of SDRAM (but some sellers might sell pre-written SD-CARDS and don't bother to update them when a new SDRAM type is used).
The solution for reason 1 is not user solvable, simply return your PI as defective.
The solution for reason 2 is to use the very latest release of Raspbian, but note that changes in SDRAM type are rare and are usually introduced in the BLOG, with some fanfare. SDRAM isn't simply changed because the supplier still had a batch of cheap SDRAM's available. If solution 2 does not work assume reason 1.