I'm pretty sure you are limited to using 4gb sticks. The user manual is very confusing (I never liked Intel-branded mobos).
Here:
I highlighted in red the relevant line. I could be wrong but I believe by "2Gb memory technology" they are referring to 2Gb per rank. A dual-rank stick from that era could have 4gb per stick. It's the only way I can see 4 RAM slots making 16gb max with "2gb memory technology" according to the manual. Anyway, it would seem that if two of the RAM slots aren't working then you are limited to 8gb. You might try reseating the CPU - those slots that don't work might be because of a bad connection with the CPU and the socket.
Here's the manual for the mobo:
https://www.intel.com/content/dam/s...ards/H55_P55/DH55TC/DH55TC_ProductGuide01.pdf