Why does the laptop not see the second RAM chip?

Question

laptops

lenovo laptop
board lnvnb161216
Built-in bar for 4 GB. We decided to add more, bought 6 GB 2420 (or something like that), set it up, it worked properly for a month. need higher. We bought a new 8 GB ddr4 for 2660. I worked for a couple of months and again stopped seeing. Sometimes it happens that he sees, but this is 1k10 switching on the laptop.

Answer 1

Google says that the mother is bad / does not support a frequency of more than 2400