What is the setting you use (if any) for the "Pre-trigger frame buffer size?" I'm uncertain as to (1) if I need it or (2) what to set it at.
Here's what the BI Help says about it...
Pre-trigger frame buffer size. Blue Iris maintains a buffer of images as they are received from the camera--when motion is detected, you may choose to record up to 50 images from this buffer so that you can see what occurred just prior to the event which actually triggered the motion sensor. Please note that the use of this feature consumes considerable memory and CPU resources--it should only be used if you have both to spare.
What is your opinion about the "Pre-trigger frame buffer size?"