There's several factors for this
1. if your using a replay you need to not be at the end (i.e there's no replay to show after the point you've entered photo mode) as then it cannot move things on instead everything just got backwards in the frame and it doesn't work how you anticipate.
2. The angle you have the camera seems to also effect how low you need to be before it starts blurring the images. As a rule 250/1 or lower it will do it there's circumstances where it will click in higher your best bet is to just try all the settings and see when it starts "rendering" and then thats the bite point to begin trying different levels to see what looks best.
3. Your speed, because the photo mode is using the replay file in someway to generate blurring if your moving fast the blur will click in quicker than if you're stopped in which case nothing you do will cause it to start blurring the scene around you or the car depending which way the setup your using. You can also manipulate this if your turning slightly you get a different effect to what you would if your going dead straight.
Basically you just have to play around with the settings see what works and accept sometimes the great idea you may have had doesn't work in practice due to the hand dealt.