I think what’s tripping you up is you may have constant motion. As Bam indicated, the motion sensor has to clear before it can trip again and command the cam to take more video. And then it’s only in 12-second samples once every minute.
If you are at your device when you receive the alert you can switch to the camera’s live feed.
If you later see the alert and have a SD card installed, and the SD card is set to continuous record mode, then you should be able to go to the time of the alert in playback mode to see everything that occurred.
Alternately, if your SD card is in Events-only mode, you should be able to do the same thing; only the camera will record motion in 1 minute increments until it ends, with no clear required.
So with continuous motion, a SD card is your best bet. A motion sensor can command a camera to do the standard 12-second alert once every minute if it’s allowed to clear. But using that method, the sensor has to clear.