Sometimes the BIOS must be reset when additional memory is installed. I don't know if there is a BIOS (CMOS Memory) reset jumper on the motherboard. If not, power off the PC, then open the case and remove the CR2032 backup battery on the motherboard for at least 10 minutes. Reinstall the battery, observing battery polarity and then power on and check the BIOS to see if all the memory is detected. You may have to reset something in the BIOS if you have changed anything from the default. Then boot Windows and see what it reports.