Left click not registering

This is not a "my mouse is broken" issue. I tested by clicking lines of text with the left click and highlighting that line for about 5 minutes straight and it did not hiccup once. However in-game it'll happen multiple times in that amount of time frame where, the left mouse clicks don't register, I can click multiple times hold it down, it doesn't matter. It just doesn't register and it's gotten me killed a few times now. Very frustrating.