Plates reflect IR much more than their surroundings so they get washed out as the camera tries to display the rest of the scene properly. Part of this is lengthening the exposure time at night to probably around 1/30th of a second. Manually set your exposure time to 1/500th or 1/1000th and drop the gain in your settings. That'll make the cam useless at night for anything but plates.
Another solution is to set the cam in day mode at night. No IR to screw up the plates, and if the cam is good enough (many aren't), you'll get good colour pics of people and cars, plus slow moving vehicles coming straight on will have visible plates. I had an Axis P3364VE set up in my driveway like this and it worked great plus snagged plates behind headlights of those coming in my driveway slowly and parking just fine. It was a dead end and had lots of light from the headlights reflecting back at the car as well to help. Cheap (and not-so-cheap) Hiks and others have a problem doing this though. This doesn't work well for vehicles that aren't at slow driveway speeds. Lots of IR with fast shutter speeds on a cam dedicated to snagging plates is the best way to go.
