Recording cutting off shorter than set length

Why is my Ring doorbell 3 cutting off recording mid action? I have video record length set at 120 seconds, but it's cutting off after just a few in some instances. These instances are the ones on my doorstep and the ones I really need to record, having been assaulted by my neighbour. It's the whole reason I got this in the first place. It records passers by til they've left the shot, but neighbours and postman etc are cut off.