Why won't my laptop utilize the full RAM?

Computers use only the amount of RAM that the programs are demanding space on, no more, no less. If you run more programs at the same time, or run more complicated programs, more RAM will be used automatically.
It is very rare that a laptop would ever need more than 8GB of RAM. Many will run fine on 2 or 4 GB if only one program (like an Internet browser) is run.
Speed of running a game depends upon processor chip, video system, operating system, and what utilities or programs are running.
Perhaps the w10 install has occupied the CPU more than the Win8 did. That would affect the performance of a game.

Normally you have 4gb stack when buyng a laptop… Windows utilize 2gb or 3gb depends… Rest prgorams utilize the remaining one… Now basic programs like google chrome won't take more than 200mb but photoshop with a large image in it will take 10 gb… So by program demands they utilize how much they need… If you run 1 game at a time will take 1gb more, but runing 4 games at once will utilize like 4gb more… I have 24gb and never seen it with more than 12gb load… To understand better think on cars lets say you have a bmw with 300hp and can go 250km/h < in everiday you won't go faster than 110km/h so rest until 250km/h is neded in special conditions when your on hurry… Same is with rams, you have 16gb but you utilize in normal conditions like 3 or 4gb rest are just in case