How come, when I am using both my MBP and external monitor, the mouse wont move over?

I am connecting an external monitor to my computer and I am using both my MBP screen and the external monitor. I sometimes have a problem shifting the mouse over. It gets "stuck" on the MBP screen and hits the side but wont shift over. I have to slide the mouse a few times until it finally crosses over into the other screen.

Any idea?

In System Preferences > Displays, check the arrangement settings of your external display. If the displays have different resolutions, you will run into this issue where the mouse doesn't move over. The mouse literally has nowhere to go since one display is "longer" than the other.