So, First Things first...
Make sure you have the latest Drivers from NVidia
http/www.nvidia.com/Download/index.aspx
Then, if your SLI is enabled but not utilizing both cards.. Do you have the Nvidia "GeForce Experience" installed? That program when enabled will automatically set your settings for each game you have installed that it can see.. if Nvidia thinks a game runs better without SLI it may disable per-game.
If neither of those solve your issue, please reply with your -
Graphics Driver number
Any Nvidia Software in your Software list..
List a couple of games that are having your issue.
THanks.