How to fix a computer not detecting all new memory
Memory not set up properly in BIOS
Verify memory is properly set up and detected within BIOS setup. If the memory is not detected in BIOS, the rest of the computer will not detect the memory.
Computer does not support as much memory as you are attempting to add
Verify through documentation or through the computer or motherboard manufacturer the computer can support the amount of memory you are adding. Many computers may not be able to support large amounts of memory exceeding 128, 256, or 512 MB of memory.
Also, the manufacturer of a computer or motherboard may have a BIOS update available that may correct memory detection issues or allow a computer to accept more memory.
Memory incompatibility
If the computer can support the amount of memory you are attempting to add, verify the new memory is compatible with the existing memory.
Because of the differences in computer motherboards, this information must be obtained either through the computer manufacturer or documentation on the computer motherboard.
Bad computer memory
If the recommendations above are reviewed and didn't resolve the issue, the memory or motherboard may be bad.