When chosen, Importance Sampling defines a target for scattered rays, redirects scatter rays towards the target, and then down-scales the power in these rays so that the flux seen at the target object is realistic. When you reduce the size or change the distance of your target, that corresponds to a lower probability of scattered rays hitting it, and therefore the effect of the power scaling on rays is increased.

If you’re finding that your target is reporting no scattered rays, it most likely means that those that would make it to the target are below the ray intensity threshold specified in the System Explorer. Rays that fall below the chosen threshold are terminated during the course of the ray trace to improve efficiency. To fix this, you will need to reduce the Minimum Relative Ray Intensity. This option is found in System Explorer…Non-Sequential…Minimum Relative Ray Intensity:

Should your target get smaller or further away, you may need to reduce this value again.