I quite like PCChips, I'm at a success rate of about 8 for 9 whereas gigabyte, I'm 0 for 3.
Anyway, could be a number of things... firstly, BIOS doesn't really, really detect RAM, it reads the presence detect bits on the last few pins of the SIMM which are encoded with zero ohm resistors or just solder bridges. The coding for IBM PS/2 SIMMs is different, so an 8MB detects as a 2MB IIRC and other weirdness. So if they've got IBM part numbers on them, you could have PS/2 SIMMs..
Secondly, those presence detect lines could be shorted somewhere else, mashed pins in the socket, debris in socket, solder bridge on PCB and socket pins on the backside of the board mashed together or touching. So check all of those possibilities.
Thirdly, maybe the CMOS RAM has gone bad or got a bit stuck. Not properly clearing CMOS can have some weird effects also, so clear it, restore defaults, retry. You may be able to confirm if a bit is stuck by using one of those generic CMOS utilities that lets you poke bits to it. if it won't change, then yup there's something up with that.
Possibly if none of these reveal the exact problem, or it's a stuck bit, you can still get it to use the installed RAM with a utility that tells DOS to use extra RAM, even if not reported. You can crash the system by telling it to use RAM you haven't got, or that's somehow cut off, a break in an address line for example.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.