Sounds like it isn't changing from the low power/general use Intel graphics to the more powerful Nvidia when a game is started.
Make sure you have both the Intel and Nvidia video drivers
installed, as the switchable graphics (or Optimus Technology as Nvidia likes to call it) won't work without them. Some of the laptop manufacturers include both in a single download (including Asus), but your model isn't one of them. You need at least the Intel driver
from here, ASUSTeK Computer Inc. - Notebooks- ASUS U45Jc
and likely the Nvidia one as well. Reason for saying that is not sure if the latest Nvidia driver for it, NVIDIA DRIVERS 285.62 WHQL
will work or not, as it also only contains the Nvidia driver and not the Intel as well (I have seen previous version of it directly from Nvidia that did).