Hi, Bob.
In the case of sparkles for this layer, it really relies on a lightsource directly in front of the camera as the 'sparkles' are actually just reflective shader spots of Perlin noise.
If you enable the sparkle layer, make sure your sun(or a separate lightsource) is directly in front of the camera. If it disappears out of the view it will only reflect as far as the spread of the reflective shader allows. If the light is behind, or out of view of the camera FOV, there's no light to be reflected.
Imagine the surface as if it were a lake object with a water shader, there will be reflection spread(of the sun's highlights) when the sun is in front and very little if it's out of view.
An awkward and not very handy method of creating sparkles but I can't figure out a decent other way to make glinting otherwise.