I have an instance that has been assigned 8 G of memory. From the init.ora:
memory_max_target 8589934592
memory_target 8589934592
No other memory parameters have been set. The database has been running since July 14th, yet AWR only shows it having allocated about 6GB of RAM to the SGA/PGA:

The Buffer Pool Advisor shows that database performance would be much better if another couple of Gigs were tacked onto it. I'm trying to think of some reason the server would decide not to (or be unable to) use the full amount of memory assigned via MEMORY_TARGET and I'm coming up blank. This is Oracle 12.2.0.1.0 Enterprise running on Solaris.