5GHz often doesn't have the range of 2.4GHz so you connect to the 2.4GHz more often than 5.
I've just moved into my new student house and I cannot always find (rarely find) the 5GHz network. I've connected to it before, but can no longer see it on my list of networks. I can see other 5GHz Networks too.
If I force my NIC to A/N/AC, it will detect the network, but fail to connect.
My NIC is: Realtek 8812AE Wireless LAN 802.11ac
Before anyone says, it's not due to range. It's within 2-3 meters.