One of 2 things:
1) The motherboard doesn't support faster RAM
2) BIOS settings or software needs to be changed to support the faster RAM.
Best to check with the company that manufactured the RAM. Faster RAM in MHz doesn't always mean better...
I would suggest using this calculator to compare old RAM to new RAM:
https://notkyon.moe/ram-latency.htm
CAS latency is an important metric to consider when building a new computer.